A 50m Pool In The Sea?

Here is a picture of a rather old pool constructed in the sea off St.Malo in France. (click on picture to enlarge)

Many Summer Camps in the USA and Canda have pool constructed in a section of lake too.

With Cayman's lovely clear, warm, calm waters is such a construction possible?

A series of bulkheads...set of lane lines...?

Just a thought.