Me and my brother have been kayak fishing for a few years, here's some advice from my experience:
- PFD- ALWAYS on, not worth the risks for some unrestricted movement
- Flag kit- especially if going into the salt or any other big water
- don't rig it or drill/cut any holes to rig it until you've fished it a couple times...sometimes what you think is the perfect spot for a rod holder or fishfinder placement ends up being far from optimal once you're on the water
- use duct seal to mount your transducer inside the hull, works like a charm, and is removable...depending on where you fish, and how much water comes in, you may be able to get away with routing the wires through a hatch or something, without drilling
I fish mainly sweetwater with my kid in my 2 person hobie, and my brother sticks mainly to the salt, can help you out with rigging advice if you need it....the toughest hole to drill is the first one, so make sure its in the right spot, lol