One would think that the generator would take on the full electrical load, but unknown how that particular boat is wired. Try the Rinker owners crew https://rinkerboats.vanillacommunities.com/
First- If it came from amazon listed as “replacement for”, put the original back on
Second- I’d be looking for a restriction in the fuel flow be it from stuff stuck in the pickup tube (90 degree turn or a screen) as well as a vent not venting causing a vacuum
Try listing your issue(s) with the motor (include year/model at minimum) and maybe the big brains around here can point you in the right direction and save you some coin
Where are you purchasing your switches from?
Are you measuring with something like a volt meter (that will read 12 volts easily without a load) or a test light with an incandescent bulb (that will draw some current)?