A major difficulty in the World of Sea is that the most common enemies, the merfolk, drop one of two weapons when defeated: the Bow and the Wand. Neither of these first-level weapons can break blocks, so you need to be careful what you pick up. Meanwhile, I'm really getting mobbed by the merfolk; sure enough, I accidentally pick up a Bow. I really have no choice after that but to keep killing merfolk until I get the Silver Arrow, which upgrades the Bow and gives it block-breaking ability.
There's a Blue Hourglass in the corner of those blocks in the middle of the screen, and once I grab it I quickly swim back to the upper level. Curiously enough, this doesn't bring us to the surface -- it's entirely water-logged up here.
I'm getting really worried now because my life gauge is dangerously low, and a couple more hits will mean Athena will leave mermaid form and be too tall to fit through the narrow gap. It's gonna be close...
The Boss of the World of Sea was an incredibly imposing figure during my childhood runs at Athena. He's less scary now that I've learned a trick. Normally hit detection works against you in this game, but on this occasion the fact that the Boss's graphic doesn't line up with his hit box works in our favor.
That area I've circled in red? If you stand right at the edge of his platform and fire arrows straight forward, you'll hit that area and it'll count as a hit on the Boss. Just keep firing, and he'll be destroyed in no time, and his projectiles can't seem to hit you.
He drops a simple Club, which is important if you have a weapon that can't break blocks. I'm pretty happy with my Silver Arrow, so it's on to the last room of the level. Next time: the World of Sky, in which I get lost and waste a bunch of time. Huzzah!