crazy idea! but it will great idea for diver people so they can navigator under water, i not know if GPS will work underwater.... if not so they can basic map without gps to plan journey....or read and surf when you bath ,-- great idea with lay and rest enjoy bath or shower <-- crazy and stupid people will do it.... waterproof will great usefully...
i would make simple of it... if i was engineer, i will put case airtight and watertight
option 1 enlarge size plenty space for pin plug (see reason later) and hold all edge and buy spare wire pin plug thought airtight and watertight case. make sure transparent screen where can press work well as well... and spare wire pin plug make it waterproof wire cover and put maybe in case battery flat.... at underwater.. if battery box bring for long time underwater (which really not need as it can through 10 hours)
option 2 build case watertight and airtight make sure transparent screen watertight and airtight and press work... it is basic and simple which battery not need charge - which i point before not need charger underwater as ipad can operate for 10 hours anyway
i should not revives this plan so you can stole my idea (by way i am not engineer anyway... and it is need patent) by way i think it will very low profit income anyway as few people will buy it for this reason... (free my plan to your welcome build it) by way tip for me will be lovely (LOL)