This looks like a comparatively large manatee enclosure. Very nice! I remember watching the transfer of these animals to Denmark on Danish TV. They lived in some type of inner city park, if I remember correctly. A very muddy pool and it was quite a job to catch them.
And you cannot even see half of it in this picture. The pool consists of a few more of these relatively shallow "canals"´and then a deeper and bigger main area. The "canals" you can see here are the most shallow and the water depth in the biggest part of the pool is more than twice as deep.
This is a prime manatee habitat! There has been four births since 2003 (unfortunately though two of them were stillbirths) and the future looks just a little bit better for the manatee population in Europe.