I have a feeling Mantine wont be a typical ride Pokemon as the ones in SM

First of all this beach is on route 2 so very early on in the game, a little to early to get a ride mon. In SM you get the ride pager and Tauros after Grand Trial. It could happen earlier in USUM but i doubt it will be this early

Second thing is that we never see ride mons in the overworld, they're just summoned. This Mantine just sits there.

Third thing is that there doesnt seem to be any diving spot, instead we have a lot of surfing boards

I think its probably some small mini-game you can play on each island (bar Poni) because these stands are on each island