The sea water was cold and it cooled the air layer in contact with its surface. However, just above it, there was a warmer layer of air keeping it stable. That temperature inversion layer then acted just like a trap for the light from the setting Sun and a complex series of reflections and refractions within it created a mirror image resembling a rectangle beneath the highly flattened residual image of the Sun`s disk. The result was that Rectangular Sun.

Nikon CoolPix P900 camera

Photoscape to reduce glare