First off: building on water should be way harder in UII due to proper building physics. Building over it should demand fairly high amount of resources and advanced building techniques, taking in mind the fact that stronger and/or larger structures would logically be even harder to build over water. Underwater building should be even harder (if not impossible).
Now, the turned could pursue you undeewater and walk in the bottom trying to reach you, but they should be substantially slowed down by masses of water and shoud likely get drowned after a couple of minutes. If not, turned could naturally avoid any masses of water they can’t get through without drowning, such as lakes and seas.
Very alternatively, there could be a type of turned that could be adapted to survive underwater, similar to Minecraft’s drowned, with the exception that this turned shouldn’t be able to throw tridents nor anything at you.
perhaps they changed it, idk. try it out for yourself in the editor. there’s that camp in the France map with a pond, the zombies can go in the water there