Absolutely fundamental to diving in the sea is an understanding of tides and currents.
This page explains the basics:-

Introduction
Divers need to have an understanding of tides since tidal movement will have a major influence on sea dives and the launching of boats.

The maximum water level is called high tide; the minimum level is low tide. The time between high tide and low tide is called ebb or falling tide, the time between low tide and high tide is called flow, flood, or rising tide. At any given point on the ocean, there are normally two high tides and two low tides each day.

On average, high tides occur 12 hours 24 minutes apart. The 12 hours is due to the Earth 's rotation and the 24 minutes to the Moon's orbit. The 12 hours is half of a solar day and the 24 minutes is half of a lunar extension, which is 1/(29-day lunar cycle).
The tides at a given place in the Earth's oceans occur about an hour later each day. Since the Moon passes overhead about an hour later each day, it was long suspected that the Moon was associated with tides. It is the combined gravitational pull of sun and moon that cause tidal flows. The moon causes about 66% of that effect and the sun 32%.

The earth rotates every 24 hours and during that time there will be two high tides (flood) and 2 low tides (ebb). However the moon is also rotating around the earth, taking about a month to do so. The net effect of this is that tides patterns change every 24 hours and 52 minutes.

Consider a water molecule in the ocean. whilst it is attracted gravitationally by the earth, it also experiences a much smaller gravitational attraction from the moon (much smaller because the Moon is much further away and much less massive than the earth).

The water nearest to the moon will have a stronger pull than the water farther away. In this diagram there are two bulges because of the differential gravitational forces.

The ocean at point A is closer to the Moon and experiences a larger gravitational force than the Earth at point B or the ocean at point C. Because it experiences a larger attraction, it is pulled away from the Earth, toward the Moon, thus producing the bulge on the right side. Loosely, we may think of the bulge on the left side as arising because the Earth is pulled away from the water on that side because the gravitational force exerted by the Moon at point B is larger than that exerted at point C. Additionally the earth is rotating under these bulges. A given point on the surface will experience two high and two low tides for each rotation of the planet.

Every 27.3 days, the earth and the moon revolve around a shared point, so the tidal pattern repeats the cycle in that time frame.

Tidal times vary due to local geography. In extreme cases there is only a single low tide and high tide each day.

In most of the world, the time between high and low tide is consistent, approximately 12 hours and 25 minutes, which is why the high and low tides seem to advance by an hour each morning and evening, but low tide is not always half way between them. In some places the flood tide rises quickly after a long period of several hours of low water.

One of the most dramatic places in the world to watch a high tide is in the upper reaches of the Severn Estuary. There the high tide rolls in on a tidal bore, a wave that quickly travels up river against the prevailing current. The phenomenon is caused by the fact that the tide is funneled into a shallow narrow river from a broad bay. This area has some of the most dramatic differences in tidal height in the world.

Tides are classified into two types; Neaps and Springs.

Neap Tides
When the moon and the sun are perpendicular to each other their respective gravitational pulls (to a degree) counteract each other. This results in weaker tides called neap tides. To the diver this indicates slower currents. In this diagram the moon is pulling the water one way and the sun another. In the UK neap tides occur about 2 days after the first and last quarter moons.

Spring Tides
When the sun and the moon are lined up with the Earth as in the examples below, large tides are experienced. These are called spring tides. The effect on tides is about the same whether the sun and moon are lined up or, on opposite sides of the Earth. In the UK spring tides occur about two days after new and full moons.

Phases of the Moon
The phases that the Moon goes through are caused by two things:

1. The moon revolving around the earth,
2. The Moon reflecting sunlight towards the Earth.

Half of the Moon is always lit, not just the portion we see: however, sometimes we only see a profile of the lit portion of the Moon. Certain phases of the Moon result depending on its orbit, and the Moon's orbit is responsible for the phase changes we see.

Since we only see the lit portion of the moon that is facing earth, we see a moon phase. There are eight phases that the moon goes through and they always occur in the same order. The Sun's light seems to move from right to left across the surface of the Moon.
The phases of the Moon are:

Spring tides do not occur in UK waters until about 2 days after the new and full moon. Similarly Neaps occur about 2 days after the moon's quarters.The phases as shown above are explained below:

A Full moon will rise in the East. A New moon will rise in the West.

Tidal Stream Chart
This is shown on an Admiralty chart and used in conjunction with magenta tidal diamonds. Details include Location, Set (direction) in degrees true, Rate (speed or drift), and time. 'Sp'. and 'Np' refer to speed and not tidal height. Also shown is the latitude and longitude of the diamond and the local port to which the tidal differences are related to.

A Tidal Stream Atlas does the same as above and gives a quick pictorial view of "Standard Ports".There is one hour printed per page and one page covers only one area.To find the set of the tide you must use either parallel rules or a protractor.
Tides Make until they reach springs and Take Off until they are neaps.When yachting you can either Stem which is to go against a tide or Run which is to go with the tide. Also if the wind is on the starboard side, the vessel is said to be on the starboard tack, if the wind is on the port side, the vessel is on the port tack.

Tide Tables
UK tide tables are usually given in Greenwich Mean Time (GMT), not British Summer Time (BST). Therefore add one hour between March 27th and October 23rd for BST.

Rule of Twelfths
There are two methods of calculating tidal height and water depth, one makes use of a 'tidal curve' and is preferred where you need to be as accurate as possible. The second method is known as the 'Rule of Twelfths' and can be easily calculated.
The level of water does not rise or fall at a constant rate throughout the 6 hour duration of a rising or falling tide, but the amount by which it will do so can be estimated mentally by means of the following rough guide:

• 1st hours rise or fall = 1/12 of Range
• 2nd hours rise or fall = 2/12 of Range
• 3rd hours rise or fall = 3/12 of Range
• 4th hours rise or fall = 3/12 of Range
• 5th hours rise or fall = 2/12 of Range
• 6th hours rise or fall = 1/12 of Range

Assume that after looking at the relevant tide tables, a party plan to dive a wreck at 4pm. The tidal range is 6 meters and the depth at low water (1 pm) is 10m.

Each twelfth would = 0.5m ( tidal range / 12 = 0.5m)