Can't say for sure one way or another without a detailed check, but the game is notorious for the IRQ being set strangely, and it makes the water display funny. While I always saw it as taking up most of the screen visually, and flickering on and off, I can't say that MESS might not display it as a single line. And that's what has me rather unsure here: the scanline is very obviously the top of the water.
As far as the 224/225 thing goes, I'm pretty sure it works like this (but it's been ages since I looked at anything SNES):
0 - sprites are processed for line 1, nothing is drawn, but this is still the origin of the tilemap.
1 - BG drawn, sprites drawn. (note that BGs are now on the second line of the tilemap, so a y-scroll of -1 is allegedly common)
224/240 - last rendered line, 240 is sometimes shorter than other scanlines.
(but that's also pre-bsnes info, so YMMV
)
