No point guessing. It’s a pity no one has commented from 3DLP in the 6+ months since I wrote my question; it doesn’t inspire purchase of designs.
It is normal practice to fit water rudders to the rear of floats as they look and generally work far better (too much flexibility in long thin bits of piano wire).
Making servos and connections highly Water-resistant is easy (nothing is ever truly waterproof) and they tend not to get much spray anyway; again that’s normal practice for anyone regularly flying from water. Servos are usually just rebated in to the top of the float section so that only the very top of the body and output protrude.