Ecco 2 is pretty much an expansion pack on the first game.
There's some new stuff that's pretty cool, like changing into other types of animals, but it's buried far into the game behind a bunch of other tedious crap.
The gameplay is essentially exactly the same as in the first Ecco, meaning the graphics are still nice and the "dolphin physics" are still unique and interesting, but the level layouts still suck and the play control is still problematic.
The main new thing is that you "teleport" between levels now with this quasi-3D bonus game thing that's all annoying and frustrating.
I found the music really annoying too.
I think Ecco was basically a good idea, I wish they had actually found a better way to execute it though, as it is these two games are really just not fun. I've never tried the later games, I wonder if any of them came out OK.