I can say from bitter experience that what is sensible way to run your fleets in SP is not good in MP. In SP I play with my fleets properly balanced, I have destroyer hunter groups, main battle fleets etc etc. In MP many players seem to group to uber stacks and you have to combat that.
It all comes down to building capacity, if you are the US player you can afford to completely lose your navy as long as you hurt the Japanese player really bad. You will be able to repair (any battered hulks that limped away) and build new faster and in greater numbers than the Japanese player, you will always win in the end. If the Japanese player groups into an uber stack and actually comes out to play, something like attempts an invasion of pearl or the like, then gleefully accept the challenge group all your ships into an uber stack and fight.
In a MP game the Japanese player must really use his fleet as a deterrent and a swift invasion support unit. He must turn up here and then be over there. They really must avoid a massive decisive fleet engagement with the US player, because win or lose, they can't avoid the loss. So you the US player group and uber stack and seek that battle, if he's a good player he will retreat from areas your uber stack turns up in, meaning you can have a effect with your fleet even when you are searching for the vital engagement. Be careful, a good Japanese player will constantly attempt to nibble at you with air and subs, so carriers actually become valuable, if they are all fighters and you have 5+ they can provide CAP coverage, a good percentage of destroyers keeps the subs at bay.
Subs are dangerous things for those that aren't careful, a lot of inexperienced players group them into their uber stack. This can be fatal, because Subs tend to loose org and strength at a slower rate than surface vessels. This means that your navy stays in the fight a lot longer than it normally would before it would hit the retreat trigger. Subs mixed with surface ships in a long engagement often seem to win, but the surface ships that were fighting along side die completely.
