Check your battery connections and your grounds first.
Also, you didn't say what boat you have? On Grand Banks, there is a factory-installed pair of relays mounted on the bulkhead that combines the start and house batteries when you hit the start button. If this fails, you are only starting with one bank which, with ancient cabling, might be a bit deficient for spinning the old clam-crusher fast enough for a good start.
I tossed mine and used a battery switch.
"...Tongue tied and twisted, just an earthbound misfit..."