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Well, whirlpools... Yes. In general, you can use a mod to make them a regular portal and enjoy the game. However, this isn't very noticeable in the fifth installment, as there are few naval maps, but...
If we're talking about who will be developing the sixth Heroes game, one thing is clear: it won't be Nival. It has been announced that Nival will now only be working on online projects, while offline development is no longer a priority. Nival is now 100% focused on online games.
And the franchise is evolving. The Chinese are making Heroes of Might and Magic Online, and an indie studio is developing Might & Magic: Clash of Heroes for the Nintendo DS. Moreover, the events of the latter take place 40 years before the events of the fifth installment. So, the series is not dead, and the sixth installment is 99.9% likely to happen. Well, I think you knew that without me.
I believe that the new race in the sixth installment will be the Nagas. The Castle will (with a 90% probability) be designed in a Japanese style, possibly with some Chinese elements. The Nagas worship the Water Dragon - the wise Shalassa... Perhaps the seas in the sixth installment will be more lively, but the race will not be a naval one. In general, the Nagas could have been included in the fifth installment in the third add-on, but its development was canceled. Well, I think you knew that without me.
We'll wait and see.