I can't say I've ever heard a particularly vocal distain for the Ohranger mecha, but I'll take your word for it.

How long take they repeat the same theme ?

There was a 12 year difference between 2 Dino shows and Ninja shows iirc

Why Toei don't make a Sea-themed sentai ?

What CAN you really do with a sea theme? Not just story wise, but also ranger wise

I think, we could have similar mecha like Ginga Bitus but this time a whale.

We just had whales and sharks in Zyuohger. Though the team probably can't think of an exciting premise or gimmicks

A full blown water team wouldn't work.
90% of the fights would take place on land since they don't have the budget to film everyone by the water all the time or to CG the rangers to look like they're fighting underwater.
It's just not feasible.

Add in that all the mechs would just fly out of the waters anyway, and you're left asking "If all the fights take place on land, and all the mechs just fly, why is it water themed?"

I'd actually love a water themed team honestly. Too bad water isn't nearly as easy to film in as land or the air.

Why the 2020's sentai will be olympic theme ?