I've got a Garmin GPS/Fishfinder combo. It has the detailed maps for every lake in the U.S. and then I also purchased the detailed map for all of the coastal NW region. The lakes are preloaded and the coastal region is on a small memory card. When I use the boat in the lake, I get detailed maps. If I go in the Delta around my house, I throw the card in and get detailed maps as well. I think mine even has aerial photos of certain harbors.
Mine works very well for "seeing" fish. If I could only get it to catch them for me, I'd consider it a successful investment.
Another thing to do is to ENSURE you have whatever safety equipment is required for the area you're going. Most people assume that life jackets are all that is needed. In reality, the equipment list is quite long. Certain boats require a fire extinguisher. I think all boats require a noise making device like a whistle. Depending on where you are diving or fishing, you may be required to carry flares. Just double check you have what is needed in case you are stopped and boarded. I look at it from the position of there must be a reason it is required. Also, if it saves my ass one day, it was worth having. One thing that is not required (unless it's changed) is a first aid kit. Obviously it would be a good thing to have no matter what the boat is used for.