Ballast tanks in submarines are normally not comparmented even if it occurs there can be one bulkhead divinding the ballat tank into two.
Ballast tanks are normally open at the bottom but there are also boats with ballast tanks that have a kingston valve at the bottom meaning that they need not be pressurised to keep the water out when on surface. This is a matter of ballast tank philosophy. You don't mix the two principles in one boat.