Experience from my most recent game suggests that the AI builds top of the line naval units relatively quickly on the higher levels, which would match up with your theory about having he cash Fisherking.

Game still not won, but starting to look promising. Sudden naval assault by France and Spain was beaten (they are both now gone as are all their former colonies) by brutal crisis management. Peace was made everywhere else and some of my territories were traded for peace or alliance. Every uneccessary army (regardless of quality) was disbanded, taxes were raised briefly and a short sharp focused assault destroyed them both on land. Had to give France away to a protectorate so that the
single truly powerful army I could afford with the blockade in place could go on to destroy Spain too.

Once I got over the panic of seeing my 30k surplus turn into a 30k defecit it was actually the most fun game of Empire I have played...