I have a 20 foot sylvan with a new battery. At times it starts right up, other times when I turn the key it acts like a low or dead battery, however all other electronics work great.
Most of the time if it has been sitting for a while it will start right up, I’ll go out and ski, kill the engine and try to start it back up and it acts like a low or dead battery. The engine will turn over very slowley.
Telling us the engine you have would be helpful....
Start by looking at the connections at the battery or batteries, the starter and the starter solenoid.
Also, the amount of amps required to run electronics and fire up your gauges is miniscule; the amount required to spin a starter motor is huge. Check the battery:
Make sure its charged and being charged
make sure its got electrolyte ( acid) covering the plates.
Is it really "new? " I am sure you just bought it, but how long did the merchant have it in stock? ( Batteries have a date code on them indicating year and month of manufcature. The letter is the month--A = jan for isntance and the number equals the year)
Did you or the merchant charge it before installing in the boat?
Your problem could also be your use. If it starts in the morning and then you go to the sandbar all day and play the stereo and run other accessories and then it wont start at end of day, you either need to get a bigger battery, add another battery or limit your use of electricity when the motor is off.
Feel free to followup with more info on this...we are here to try and help.