Again, for the most part, combat on or under the seas is basically as normal.
Units that are the surface shoot each other as normal. Unless there are islands or coastal areas present, they will be almost no blocking terrain save for the units themselves.
Combat under the waves, however, is a little different. Water is considered blocking terrain, although no spotter is needed. It does mean that indirect fire is not possible, and unless the unit is firing a laser or torpedo, an attacker will only be only be able to fire at an undersea unit that is one hex away or surfaced.
Any vessel that is currently under the waves treats every hex between it and every other unit as underwater.
Whilst hovercraft may travel and fight on the water, they are slightly more vulnerable than normal vessels. Any immobilisation result is effectively treated as a destroyed hit, as a hovercraft cannot stop on water without sinking, and a sunken unit that is not equipped for undersea operations is a dead unit.
Any sea going vessel or aircraft may mount torpedoes instead of normal rockets. The torpedoes will have the same statistics as normal rockets, but they can't be fired at any unit other than an undersea, ocean going vessels or hovercraft. They have a weapon code of 'T'.
Against surface vessels, these function as normal rockets. Against undersea units, water doesn't block LOS, and thus torpedoes will effectively function as normal rockets.
Being treated as rockets does mean that a unit hit will get the +1 to the armour resistance chart roll.
Mines are another hazard that surface and undersea vessels face. Any unit that enters a hex which contains a mine must make a normal to hit roll, with a basic target of 7, with a modifier of + the size of the unit.
So a size II patrol boat entering a mined hex, gets hit on a roll of 9 or less on 2D6. If the unit is hit, then it must take a normal roll on the armour resistance chart.
Unless placed in deep water, a mine will not effect deep water units, and deep water mines will not effect normal undersea or surface vessels.
Laying mines is easy. A unit that carries them may lay one counter per turn per firing action.
Physical combat underwater is handled as normal physical combat, with the following differences. Only units at the same level as each other may conduct physical attacks, so deep water units may only attack deep water units, but not undersea or surface units. Surface units may only attack surface units and so on.
Sometimes a unit wants to get deeper than normal undersea operations, and in this case, it may get go to what is classified as deep water.
Deep water is much deeper than normal undersea, and thus may not be available in all scenarios. This, like shallow water, should be decided before the game begins.
Getting to deep water costs a +1 MP to enter, and once there, all normal undersea rules apply, with the additional combat notes.
A unit in deep water may target units in deep water with no additional penalties. Deep-water units may attack units in normal undersea or on the surface with normal range penalties, and an additional -2 modifier. Surface units target deep water units at the same penalties, but as deep water and undersea blocks terrain, a unit may only attack with lasers and torpedoes, not MG, PF or R weapons.
Storms are a hazard to naval units, and indeed any unit that crosses the waves. For the purposes of these rules, storms are handled very abstractly.
There are two types of storms, light and major, and these should be decided upon before a scenario. Storms generally take some time to brew, and there are always warnings that the various forces can sense (visually and electronically, although if the unit is currently being jammed, then this option could be out), so forces should usually be given the option to avoid those areas if at all possible, which of course it may not be.
Light storms are not a major hindrance to vessels, more of an annoyance. All surface units lose 1 MP from their movement automatically. This is lost before the unit moves. In addition, all size I and II units on the surface, and all flying units most roll 1d6 in the initiative phase. On a roll of 6, that unit takes an automatic roll on the armour resistance chart, with all units that would be automatically destroyed (such as the Neuspotter) rolling on the 'C' column. All combat rolls have an additional -1 to the roll.
Underwater vessels are not affected at all.
Major storms are not generally travelled in lightly. All surface vessels of size IV lose 2 MP. All other vessels lose half their MP (round up), and must make a 1d6 roll in the initiative phase. On a 3+, then that unit must roll on the armour resistance chart. Aircraft roll directly on the armour resistance chart.
Underwater vessels suffer the same ill effects as a light storm, while deep-water units suffer no effects.