I think the closest sea-themed Sentai is Gokaiger because Gokaiger has pirate motif despite they actually space pirate. And the most of Gokaiger opening scenes set in beach.

Sea-themed super sentai will be interesting. Maybe various naval ships can be used as mecha.

Hmm....the mecha will be similar to Daivoyager from Boukenger.

You edited the post and yet the question is still terrible as it sounds like you have advanced knowledge of the future.

Seriously dude, you ask so many inane poorly worded questions with no contributing factors. Why?

Once awhile, you guys always talk about a Olympic-themed sentai in 2020.

It's because the 2020 Olympics are going to be held in Japan. The whole country will be celebrating, so it's not out of the question to speculate that Toei might do something to capitalize on it.

Emphasis on speculate since, at the moment, an Olympics themed sentai is just a possibility.

An Olympics themed Sentai isn't gonna happen. The Olympics are so protective over their brand that I doubt Toei would fork over the cash (Or even afford it) just to use it for their Sentai.
The Olympics wouldn't see it as free promotion. They'd see it as stealing their likeness.

A sports themed Sentai might happen in 2020, but explicitly Olympics? Nope.
(And they guard their 5 ring logo like crazy, so don't expect to see that, either)

A Sports Team Sentai based on Sports could be cool. Have the Red Ranger be Baseball since Baseball is very popular over in Japan the last time I check.

Heh! They guard their logo, sure, but a Sentai whose colors just happen to be blue, yellow, black, green, and red wouldn't so much as raise an eyebrow. Not that I want to reinforce baseless speculation.

What's kind of mechas a sport-sentai team should and could have ?

To be honest?
It would likely just be more animals.

Hmm....I think sport-themed sentai mecha also can be humanoid athlete mecha.