Depends on how the water comes into play. I'd have no issue with water being in play on EVERY hole, but I wouldn't want a 300-plus forced carry off every tee (can't think of a geographical location where that'd even be feasible unless you just went back and forth a 300-foot-wide pond 18 times!). If it's a water-heavy course, obviously variety will be needed as with any land feature like elevation and woods -- so a course where you had a long water carry, then maybe water to the right, then maybe a pond in a landing zone, small creek crossings, and so on, would be acceptable from a design perspective (albeit frustrating for most levels of players).
So to answer the original question, I wouldn't want to see more than 2 or 3 "max drive" carries (excuse the ambiguity) on a course, even if each one somehow managed to be unique in its own regard (i.e. a fountain in the middle of one, an optional bailout island or peninsula on another, etc.), simply for the fact that those particular risk/reward shots are not popular with most recreational players (and aren't executed with consistency every time, thus increasing the risk of lost plastic per round).