The following are spells for wizards who need to defend against water borne attackers, be those boats, mermen, swimmers, or natural creatures. For some of these effect there are several ways to reach the desired outcome, so a few similar spells might be included.
Drown the Floating Menace
Muto Aquam 25 / Auram, R: Voice, D:Momentary, T: Part
This spell alters the water in the target area so that it is briefly transformed into air, and then quickly back again. Anything within the target area will fall as they lack the support of the liquid.
If the spell is targeted beneath a specific spot, such as beneath a particular person or boat a targeting roll is required. The effect is designed to affect a cubic area, with a volume of up to 56,000 cubic feet of water (roughly 36 wide x 43 long x 36 ft deep). The difficulty of the targeting roll should be adjusted accordingly.
(Base 4 change a liquid to gas with Au req, +2 size increase, +2 Voice, +1 Part)
And a variant using Perdo Aquam.
Swallowed by the Sea
Perdo Aquam 35, R: Waterway, D: Momentary, T: Part, Atlantean
A large amount of water is destroyed; if targeted upon a ship it will drop into the hole and be crushed as water fills the void. It looks as if the sea opened its mouth and swallowed the ship whole.
If the spell is targeted beneath a specific spot, such as beneath a particular person or boat a targeting roll is required. The effect is designed to affect a cubic area, with a volume of up to 56,000 cubic feet of water (roughly 36 wide x 43 long x 36 ft deep). The difficulty of the targeting roll should be adjusted accordingly.
(Base: 5, R: Waterway +3, D: Momentary, T: Part +1, Size +2)
In design the spell is tricky for two reasons. Firstly the Base 5 is for greatly reducing the volume of water instead of destroying outright. Leaving an amount (be that a tiny fraction, or even a third) of water behind after the spell takes effect is moot when the overall effect is placed, and the base is smaller.
Secondly the choice of Range: Waterway was used here because it allows targeting of a craft that can be perceived upon the same waterway (according to RoP:M p 43), and certainly includes Sight range.
Perfection of Tidal Wrath
Rego Aquam 40, R: Sight, D: Concentration, T: Individual
A tall churning tidal wave is created from a body of water. Once invoked the wave may be directed by the caster to crash upon specific targets, change direction, or flow back into the background water source.
At maximum the wave is ten miles wide and 100 paces wide and 15 paces high. The wave will decimate coastlines and wreck swimmers, vessels, or creatures caught within it.
(Base 10, +3 Sight, +2 size, +1 Concentration)
And finally, for a true crushingly powerful spell which is boosted (from the Ars Magica Core p125) to horrible proportions.
Neptune’s Wrath Unbound
Rego Aquam 50, R: Sight, D: Momentary, T: Individual, Ritual
A terribly large wave is created in a very large body of water. Once invoked the wave is uncontrolled, and needs 15 miles of water surface to gather into full height. At maximum the wave is ten miles wide and 200 feet high. The wave will destroy coastlines for miles inland, and potentially wipe clean islands or flat countryside.
(Base 10, +3 Sight, +5 size, ritual for large effect)
It is unlikely that a magus would actually need this spell… All the new spells for Ars Magica can be found here.