The risk in using big ships is not only in the possibility of loosing them but also in in having Levi's cast on them after a raid.
Some of us prefer not to raid for ships but rather for the gold coin the larger traders carry, casting two or more 2 HN's. Smaller raiding ships are used due to the risk of Levi's being cast which would zero any profits.
Port blockades or attacks would be an ideal use for MOW's or SOTL's, also as escort ships to trading fleets.
Perhaps increasing conflict between nations would be a nice way to do this and reduce boredom. This is a rough idea which may need some more thought:
1)Reduce the tax earnings nations get from the port population alone and increasing tax earnings through trade ships selling and buying at certain ports. This could force larger nations to compete for profitable ports through blockades and constant attacks on trade fleets.
You could also play around with the relation of port population levels to the ability of traders to sell at certain ports, forcing more active port management. Smaller port population would mean less goods being sold or bought therefore less tax income.
2) There seems to be a mismatch between guild & nation loyalties. One guild may have players from several nations. How about having the guilds within nations? For example: A Spanish player can only choose between guild A, B or C which consist of Spanish players or create his or her own within that nation. Strong nations would be able to coordinate their guilds to the benefit of the nation.
Smaller nations could align themselves to a larger nation, perhaps agreeing to send their fleets to certain ports securing a small percentage of the trade taxes or even discounts on trade. This might be a way for nations that don't hold ports to increase their treasury. Alternatively nations assisting others in blockades or wars could be rewarded with direct payments to the treasury.