Still not sure I see it, ok with 4 or 5 sea resources its probably worth it but that doesn't happen very often. With 2 resources, you're talking 250 hammers to give 2 hammers a turn back, plus a 15% bonus on naval units. Assuming your city's starting production is, say, 20 (which is probably on the generous side for a coastal city) then that's 3 extra production per turn when building ships. So, depending on your propensity for building ships that's going to be between 50 and 125 turns before it has paid for itself and starts providing net positive benefits, which is just too long for a mid-game situational building in my opinion. Pretty much the same opinion with forges, although being earlier they at least have a little bit more time to become useful.