We had this situation in a game of Small World tonight. Seafaring Tritons took over the lake (using two race tokens) and then removed one of them during re-deployment. There was an adjacent Sorcerer player and we wondered if he could use their special ability to take over the lake. We ended up not doing it, but it seems to me that the Sorcerer's special ability would allow them to replace the single Triton there and conquer the region. Would that work?
No, you need to be able to conquer the adjacent territory. Only races with Seafaring can conquer the sea or Lake Regions. The rules state on the Seafaring ability (page 11):
On the Conquering a Region section (page 4), regarding what conquering means.
Following Conquests, showing that you must have conquered a region to occupy it.
And finally, the sorcerer's ability (page 9)