How good are the gyms on aircraft carriers?
Depends on how much the carrier cares about them based on the two carriers I've been on.
The first one I went on had a super-fit, super energetic fit boss that held lots of classes every day and even in the evening (midnight spin outside was always cool) in the hangar bay and the "seaside" gym and were pretty popular for almost everyone.
The weight gyms, in general, are the "iron gyms" of the past mostly - just grime and sweat on top of a variety of free weights and pull up bars. Cardio gyms are a mix between rowing machines, a couple ellipticals, treadmills, hamster wheels, and spin bikes and can vary from location.
What matters more is if the gym has khaki hours in the gym and if your schedule will line up with that, or finding ways to work it into your schedule. Usually, super late at night is good or super early in the morning, like 0400-0600 is good.