Suunto 9 should have great open water track.
However that depends on the way you swim. The watch needs to come out of water so freestyle is the recommended style. Breaststroke will keep the watch in the water for long.
as a quick test , switch to navigation screen when swiming and check if the arrow that shows your location and track work while swiming. The more you keep it underwater the longer it will take to get a fix the next time you take it out