How to rule this?

[Inevitability]

My players are about to enter an undersea shipwreck guarded by a coalition of sahuagin and warforged. These guardians will attack as soon as they see the PC's approach.

I was thinking about using a sahuagin wizard Polymorphed into a giant shark (for swimming speed) together with two warforged gladiators, but I was wondering something. You see, the monk currently possesses a Trident of Fish Command, and I am not sure if it would work on the wizard. He is technically a 'beast', but does his swimming speed count as 'innate'?

So, good playgrounders, would it work?

[Townopolis]

This is tricky. I think the biggest danger is of the players coming away feeling you pulled a cheap move as DM. This is dependent on your players and their experiences, though. I know that my own experiences mean that if any DM (aside from my current and one other) threw a swimming beast against me and then suddenly it was immune to my trident, I'd call BS, and later revealing that "Syke! It was really a sahuagin" wouldn't help. Your players, on the other hand, may be different, or they may just trust you.

Two things I would probably do if I did decide to have the Sahuagin be immune:

Have the sahuagin transform within sight of the party. This way, the reason it's immune to fish command is apparent before the fight and it doesn't appear like you're backpedaling or coming up with an excuse after the fact.
If the party has a druid and/or wizard, make sure the ruling applies to them as well. I would do this with either ruling, actually.


On the note of PC druids and wizards, ruling that a caster who transforms into a beast is immune to beast control would probably work out in the party's favor in the long run if they do have transformation capabilities of their own, just be consistent.

[Strill]

Polymorph alters the target's mental scores, so I'd say that yes it does work.

[xyianth]

Due to the way the polymorph spell is worded, the polymorphed wizard is actually a beast with an innate swim speed. (it inherits the game statistics of the creature) I'd therefore say that RAW, the wizard becomes susceptible to the trident while polymorphed. You are free to rule how you like though.