Is there an dhessive or sealant thatcures under water?

Lots of epoxy resins cure under water.

Ian G.

Hydraulic cement, too.

depends what you are trying to do

Silcone sealants (single and multicomponent) can cure underwater but
physical characteristics will be weaker than when cured in air and
cure time will generally be much longer. Adhesion is possible
depending upon surface type and cure chemistry of the silicone. Try a
one part tube of acetoxy adhesive (gives of acetic acid during cure
that smells like vinegar). The other two recommendations will also
work ie epoxy resins and polymer filled cement tile adhesives.
Adhesive selection will depend upon the specific application.