Add those areas revamped as shattered and sunk areas, as a new dungeon where to go in water breathing potion, with sea creatures.Easily doable:
Recover the old areas, give them undersea features, submarine lights and encounters...
Add them with a transition area like a Cliff where it was the access to Lemur/Soulhaven Monastery.
EASY OPTION: At least... if the above suggestion were not suitable...
Please, add a Cliff area, even if a small one, where there was the Soulhaven monastery, to let see the Sea and make obvious that there is not longer Soulhaven or anyother area but the sea... and some rocks above the waves as the peninsula got collapsed...
This may require a quarter of work doing the single coastal area...
EVEN MORE EASY OPTION (Not requiring any added area at all)The current Skull Crags area leading to Guldorand and from where one could go to Soulhaven... Modify the right side (eastern side) placing there the cliff in the border, so travellers could spot the sea and understand the old monastery peninsula got collapsed and sunk...
The map area could be resized on that side to add the cliff instead keep the artifical invisible force wall without transition.
LORE:
Then, how to explain the new monastery, the new weatherstone... the new grotto?
1. The new monastery was carved in Guldorand after the monks translated them there, before the imminent disaster of the peninsula collapsing.
2. The weaterstone got moved by powerful magical rituals performed by the Mountain Druids...
3. The Spriggans moved to a twin grotto hiden untill recent times in the northern Skull Crags area.
There is no reason to not keep some sensical explanations about what happened... and leaving tracks of the old areas, not only gives some realistic flavour... also allows the addition of some new dungeon /stuff easily added.