Lead acid has something like 1/4-1/3 the energy density, higher self-discharge rates, are more expensive, and are worse for the environment. The advantage is very high power density (nice for starting cars) and rechargeability, neither of which would be useful here.

Not a hardware engineer, but I'm going to wager that it has something to do with failure rates.

Modulo correct wiring, if you have a massive stack of D-cells and one dies, it's not a big deal. If your sole source of power dies, then you either have to send out a repair crew (expensive!) or have no data until the buoy is replaced on-schedule.