Before you do that, keep in mind that electricity, is physics. You have a circuit, if you have too much resistance, you don't have enough current.
https://www.dropbox.com/s/k5qgh23sbb4fp ... icture.jpgLOL.....
If your battery cables are OE, go to the back of the boat and remove them and throw them in the trash. Go to a West Marine store, or any good marine shop and have them make you up new marine quality cables, tinned wire not the automotive quality crap that FW installed. Minimum 2 gauge for batt to starter, batt to ground. Make sure you install the cables on the batt with nyloc nuts not wing nuts. Clean the positive terminal on the starter solenoid where the positive cable from the battery connects. The starter solenoid terminals are often rusty because the drain for the manifold is right above them. Lastly take a look at the connections for the trim pump itself. If there was corrosion anywhere that is your issue. If you are not having chronic dead battery problems, the alternator is not likely to be the cause.
High resistance in your wiring, causes a substantial voltage drop and that reduces current to the ignition system and there you have it. Fixed that like I said above with a new 2 battery system with good wiring. Throw the OE stuff in the trash...Your boat is as old as mine....if its never been re-wired for the batt wiring...its WAY overdue....
And if you're going to do that, may I suggest that you take the opportunity to put in a 2 battery system with a switch. Two group 27 dual purpose batteries and a good quality switch like a Cole-Hersee, Guest or Blue Seas.