It depends.
I've been in Nassau many times (Atlantis, not on a cruise), different weeks between the first week of March and the last week of April. The ocean water will vary between brisk and comfortable, depending on the winter's weather, el nino, etc. The later in March in March you go improves the chances of the ocean water being warmer, but there's no guarantee. All of Atlantis' pools can be heated and they're usually a decent-enough temperature.
But your overall comfort level varies much more with the air-temp and wind speed, and in early March you can have days in the high 60s or days in the low 90s. It's usually a nice high 70s, low 80s, but it just varies week-to-week, and day-to-day.
we've been going mid-to-late April the past several years, and we've had reliably good weather for swimming all those years, except one year when a cold front sat over Nassau for two days and rained and was cool. But once it passed, everything got comfortable again.