I am not saying anything about the cost of Steamboat or whether it is too strong or not, but a few things should be considered here.
First of all, this is supposed to belong to Seaside. Seaside has quite a lot of cards that enable some drawing at the start of next turn: Haven (indirectly), Native Village, Warehouse (no net drawing, but a good chance to find the best card when starting with a 1 card hand), Caravan, and Wharf; and Outpost and Tactician seem also to be good. So, from that point of view in a Seaside-only or -heavy game, Militia-type attacks should be handled well more often than not. In Kingdoms where there is no good compensation for such attacks, I wouldn't buy Steamboat.
Secondly, there are a lot of deck inspection and digging cards around in all the expansions, likely more than Militia-type attacks.
Just my 2 Cents.