for Anne and FW to wait until he was in a better position to marry. Sir Walter had absolutely refused to give Anne any money- and FW had spent all his prize money, which I doubt would have been enough to support them in a genteel lifestyle anyway.

So, the choice was between a long engagement and marrying immediately, living in poverty and losing the genteel status (- an unfair thing to ask of Anne). Besides, the only way of advancement in the Navy and gain prize money at his rank was if they were at war, and there was no saying when there may be a war next.