See attached photo which shows the buoyancy chambers. If they've drilled the deck above these to secure the battery down with out sealing the holes chances are they are full of water if the deck has been sat full of water for any period of time. Draining them won't be a simple task. Will inevitably involve drilling a hole somewhere and resealing later.
Does the boat sit low in the water at the stern?
Unless they changed construction in later boats there's no reinforcement under the deck apart from these two chambers providing some structural strength. To be honest the deck is that narrow it doesn't need any reinforcement. When I rednecked my boat I blocked the flooding hull so didn't need the buoyancy chambers so I had space to fit some stringers etc under the deck. See here Searider rebuild project
RYA Instructor, Trainer, Examiner and Skipper