Not sure on this occasion sea temperature has anything to do with the nightime min.
I see Southend has 19.6C SST atm.
I am 1km from the coast here in SW Devon and the SST is 17.5C atm.
Last night's high min here I think was due to windless conditions
after a day with a max of 30.6C.
Len
Wembury
SW Devon coast