That's what I use, Costco dual purpose, Starting/Deep Cycle batteries. I believe I also have the 700CCA or 800CCA. Reasonably priced, Costco Batteries also perform up to par. You also want to get the largest CCA you can afford that can fit your storage place. Note Cranking batteries offer higher starting current but can also be damaged when discharged below 10.5 volts. Meaning if you leave them in the boat during off season and they self-discharge so low, it will actually permanently damage the battery and will not hold anymore charge. On the otherhand, a deep cycle battery may not offer higher (albeit enough to start regular 4 cylinder Force engines) starting current, but it can survive a full discharge and will revive itself as soon as it gets charged. The combination Starting/Deep Cycle batteries give you the best of both type.
Even with pure deep cycle batteries, you're still better off than having starting batteries on your boat or RV. Deep cycle batteries rated at least 600 CCA can reliably start a 125 HP Force motor. Starting batteries are more designed for cars and trucks especially during cold seasons. Unless of course you also use your boat during cold seasons, there is no need for starting batteries on your boat.
Force motors like yours require no external power to run except when starting. As soon as it is running the motor is self sustaining electrically. It does have a charging system for charging the battery and also for aux equipment. With two batteries, the tendency is to dedicate one for the aux and the other for starting. Personally, I prefer not to dedicate any battery but instead only use one battery at a time, i.e for starting and also for aux. This means as long as the motor is running it continuously charge the battery in use. Now just in case the battery in use gets drained, then you know you have a problem but at the same time you know you also have a fully charge battery for a reserve. In this situation, it means it is time to turn off auxiliaries and head back to port. Note the charging system is not really that great for it is only rated at 7 amps max which is about 85 watts. Finally, make sure you DO NOT BUY THE MAINTENANCE FREE TYPE BATTERIES. the reason for this is because the charging system is not regulated.
