The ferry is not a bad choice from Fukuoka to Seoul. The hydrofoil ferry to pusan takes about 3 hours, and then you can switch to KTX high speed rail to Seoul (there is a shuttle bus for the short ride from the ferry terminal to the train station). The train ride also takes about 3 hours. All in, the ferry + train combo takes about 3 or 4 hours longer than flying (remember that if you fly to ICN, you will have to take a limobus into town). When you're searching for FUK-ICN flights, have you tried booking a round-trip? The fares are usually cheaper than a one-way and you can just throw away the return leg.

Also, when you are in Fukuoka, don't miss the ramen stadium at canal city.