Not quite dinosaurs but close, HLBS Co have released a range of deep sea divers and ocean dwelling creatures, including several prehistoric marine reptiles.
There’s not much else available for 25/28mm scale time travellers who like to get their feet wet.
HLBS Co used to have a Tylosaurus and an Elasmosaurus as waterline models and DZ MIniatures had a very large but utterly fantastic waterline Lipleurodon. If you see any these for sale you should first grab as many as you can and secondly let me know so I can grab the rest.
Steve Barber Models do some cavemen mucking about in the water and a Carcharadon Megalodon (rough translation “fecking huge shark with fecking huge teeth”) to gobble them up.
And Flagship Games have a Ictheasaur (sic), that at 8″ must represent one of the 15m giants such as Shonisaurus.