It does not. Semi-Aquatic incorporates the "Amphibious" keyword, only with the caveats that they take damage when not near or in water, as well as losing one speed, but gain speed instead, when near or in water. Arguably, Semi-Aquatic is better than Amphibious, unless you really can't get some water generation.
theres really no downside to semi aquatic, for the -nora modifier it brings. But I don't think it is really overpowered in and of itself at the moment. I guess they wanted to make it a low impact ability since you never know if you will get water in the map or not, which is ironic considering the advantages fire decks have on lava maps.
While it doesn't affect champs in a (mostly) full semi-aquatic BG, it is a downside when there are only one or two semi-aquatic champs in the deck.
Ah that's probably true. Still, you can hold the card until u have a water gen or font battle. Almost every fs deck uses the collective and drown. I might overestimating that as well though. Hard to judge how powerful fs bonus is as a FW player.