Looks like rough water testing. The F2Y Sea Dart (part of the same strategic seaplane force programme that spawned the Tradewind) went through a programme of taxiing in progressively bigger seas to determine the operating limits, and I'd imagine the R3Y was subject to something similar. Whether this is a landing, an attempted takeoff or simply a very heavy pitching during high speed taxiing I'm not sure.
Quite an interesting programme. There was a bomber (the Martin Seamaster), a fighter (the Convair Sea Dart) and a transport (the Tradewind) intended to operate as a unit out of remote bays as a mobile strategic deterrent just like SSBN submarines. It would be supported by repurposed escort carriers, converted cruisers or submarines. It was overtaken by Polaris but there was a period when the Strategic Seaplane Force was regarded as the more viable solution.