The ornamental design for a under watermountingbracket, as shown and described.
FIG. 1 is a perspective view of an under water mounting bracket;
FIG. 2 is a front plan view thereof;
FIG. 3 is a top end view thereof;
FIG. 4 is a bottom end view thereof;
FIG. 5 is a side plan view thereof which is the same as the other side (not shown); and,
FIG. 6 is a rear plan view thereof.
The demarcation indicated by the _.._ line depicts the boundary between claimed and unclaimed environments. All broken lines form no part of the claim.