I would understand if you have a big ship and massive sonar and scan the whole bay and close offshore 3D and sell of the map but why are you trying to invent the wheel again, all above you mentioned is already done countless time in the past and current.
For an average person to buy your possible fishing spot gps marks, you need to be some sort of fishing guru, credited on publications, tv, etc...
otherwise doesnt make any commercial sense
Most of this info is already available free online.
Facebook is full of fishing community and apps, FishBrain is an example, instead of single person like you with this app you can get updates from whole community.
Keep up to date what everyone is catching, rough area, size, etc..
You can even pay for premium app $2-$3 and show you exact spot where the catch was.