I have an Apple Watch Ultra 2 and Open Water Swim tracking is terrible

Using the watch for other activities and the GPS tracking works just fine - but when I start an Open Water Swim session - the distance tracking goes crazy, normally showing random swim lengths while I am wading into the sea, once showing 460m in a 10m actual wading distance - and this is while the watch is above water! When I review the session on my iPhone, the map of the session is correct - but the distances never are. My wife has a Series 9 Applle Watch and it works fine in Open Water sessions, our iPhone session maps are the same if we follow the same course, the series 9 has correctly logged the distance and the Ultra 2 has been out by 2km on one occasion. What is the problem? I have recalibrated the watch just in case, but as I mentioned walking and running work fine, but put it in Open Water mode and it goes nuts...