My 90 yammie yours would be a bit harder to pull start

 Just disconnect your black negative lead from the flat batteries, and connect the jumper pack black cable to the loose black wire, red to any red terminal on the battery - provided it is a 12 v system, and it will work fine. They have High Capacity batteries in them, that create high current for a short period, so providing you dont need to crank too long it is fine -  and should easily start the engine, and run you back home. Be aware that you can cook your alternator if the circuit breaks or the leads come off on the way home and the battery is disconnected with the engine running though, so tape it on for the bumps.

Try it out at home..

Dont connect it the wrong way around and it will be fine, no fuses or anything blown.

Would work any of your electronics for a few hours too..

Dont connect it to the flat batteries though, as they will drain the jumper pack of the power to try and recharge themselves, and you could sit with 3 flat batteries.

Most electronics will work on a flat boat battery too, and the likleyhood of both batteries failing at the same time is very rare, so if you choose to leave the jumper pack at home, and get stuck, disconnect the 2 batteries from each other, and the better one will work all the electronics, as the dead load will be out of the circuit.

PS. switch the key to off before connecting or disconnecting the batteries, and be aware of sparks if your battery is near your fuel tanks..

You could also see if your GPS/Fishfinder has a low voltage alarm, and set this for 11.5 volts, so it will warn you early, and should still be able to start up and recharge, or get home..

Hope that helps.

Haven't had to use my jump pack on the boat yet but have used it on several other cars. Just connected straight to the flat batt terminals and started as you would normally jump start a car - the flat batt would take a while to flatten the jump pack as long as you don't muck around too much you'll be fine.

Ignition off and connect + first and disconnect + last

QR as a sparky this is how I would go if you have a battery issue , put the jump pack straight o the battery that still has some power in it , wait three minutes , first call sea rescue and let them know your location ( GPS ) in case it all fails and you don't get going , the battery pack generally has a 50 to 70 amp hour unit inside so it wont start the motor directly not having the big capacity to turn over the motor , so on the battery and then try to start your motor, hopefully all starts and then inform sea rescue your under way IMO

 agree with Paul and JDI except that, if the battery is flat because a cell has failed then connecting the small jump pack will drain it fairly quickly.
It is always a good idea to try and acsertain WHY the battery is flat if possible before trying to start it.

Check terminals, feel the battery for bulges, carefully check if it has the plates covered if poss.
Because if you manage to start it with the jump pack without finding a fault, it wont be long before you are right back at square 1, minus 1 
EFI engines draw a considerable amount of power to run the electronics, and if the charge system itself has failed you'll be unlikely to be able to go far unless maybe you have a solar panel.