We were moored at Stoke Ferry a couple of weeks ago. There is a road crossing the river which is busy with lorries, so if that is close by it would put me off. There are quite a few long term moorings there which are owned by the neighbouring caravan park. Speaking with staff on the caravan park, which backs on to the river, they say they very rarely get flooded.
You can wind at the end of the navigation about 10 minutes further on, but it can be a bit tricky if there is a strong flow. The river does get narrow before the winding hole and I doubt if you could get through with a widebeam.
You have a trip of over 2 hours to get to the Great Ouse and about another 15 minutes to get to Denver Sluice.
The Wissey is a very nice river to cruise and typical of the area very quiet. We spent 3 days on it and only saw about 5 boats.