Thank you very much for your quick responses. The good news for me is that simple patching would be enough. I thought that use of anchors or something would be overkill in this kind of case. But with my usual luck, you never know. :-)

The crack is in the lower part of the spa seat. It is not really visible from there. The spa is currently almost empty so I have access to the crack.

The epoxy path sounds suitable in this case. Would it be enough just squeeze / inject epoxy in the crack and not grind the crack all open? Any references to exact brand of epoxy that works and would be available at Home Depot or Lowe's?