How to win at Gunboat
1. Win in your half of the map. If possible, make yourself as small a threat as possible to your neighbors, and hope they start tearing into each other. Then pick on the losing side, grab what you can. Wall out the victor the best you can, but DO NOT fight him, it is in neither of your interests to do so. At this point you probably have around 8 centers.
2. Hopefully you have that done by 1905-06. Now find your nearest neighbor in the other half, start pushing stuff at him. He's involved in his own war with someone else, so this will turn the balance on that half against your new victim. But you're there at the same time as the winners on that half, so you should be making gains. At this point you should have around 10 centers. Ideally the other guy who won in your half has about the same.. Preferably he has one or two more but in a slightly overstretched position. Also, there are 4 people left on the map - two of you, 10-11 centers each, and two of them, about 6-7 centers each. Now's the fun bit.
3. Make the other 10 center guy try to stab you, and defend. Wait for the other two to go all 'Kill the Solo'er. Surreptitiously sneak three or four centers off the other guy. Create a power difference between the two 6-center people. Try to make sure one gets more centers than the other while cutting up the 10-center person. Induce a stab between them, either out of a misguided solo attempt, or frustration. This is the part that requires the most ingenuity, because you're the SC leader at the moment. It might even require pulling back and actively yielding centers to the leading 6-7 centerer to make the other one think you intend to two-way draw. All through this you should be hovering around 14 centers, sneaking your forces into position. Don't amass huge armies near the line, that'll scare anybody. Focus on getting a unit across the line, under whatever pretext necessary. A southern fleet through to the MAO, or a northern army in Galicia or Venice is an almost certain victory.
4. Win.