They could always make the dolphin be able to levitate if it is water/psychic. I see no reason why a dolphin is impossible. The creators know starters need to be mobile on land so, if they make a dolphin, they'll make sure to give it something to keep it on land without problems.
Still not convinced with those drawings, though.
Oh I'm absolutely skeptical of those drawings. And I don't discount the idea of a dolphin pokemon, it's something I've wanted since "BRAND NEW POKEMON" became a thing back in the pre-GS days. I just don't think we could have a dolphin starter. I'll absolutely eat crow if I'm wrong but I just don't see them doing it. Especially if they're banking on it levitating to sell it, but since it's the starter it wouldn't have the ability Levitate it would have Torrent, with Levitate being relegated to it's hidden ability.