We planned to go up to York (and beyond) from Selby in July earlier this year (60ft nb). The evening before we checked in with the lock keeper and sorted out the time that he would be ready for us in the morning. We turned up, a little early in case, just as he was opening up his cabin and checking his messages. He then discovered that the keeper at Naburn (who does different hours on account of the difference in tide times) had left him a message from late the day before that the moorings at Naburn were under water and that, therefore, they would also be so at York.
In consequence he strongly 'advised' (he does not have the powers to forbid) us not to attempt that day and that it might be a few days before the water level dropped sufficiently. Other boaters, one from the area, tried to tell us to ignore what he was saying and that we would be ok. As somewhat risk averse people and mindful that our insurance would probably be invalid if we traveled against the advise of the lock keeper, we stayed put. He did give us permission to over-stay on the closest moorings!
Instead we took the opportunity to visit York by train - both bus and train are convenient - and so we were able to take a closer look at the moorings. Indeed, the main stretch was well and truly under water and although at one time we did see a larger cruiser tied up there it would have been quite a leap from a narrowboat to dry land! In addition, the moorings are stepped so considerable care is needed as the water level drops.
However, the moorings adjacent to the (noisy - busy) pubs was still available and one or two boats including nb's did arrive but the amount of room was limited and, if anyone arrived when they were full, there was not much of a Plan B. The water level was high but nowhere near to flooding these moorings. The water level was high (which makes it less of a ladder climb!) but the flow rate did not seem unduly problematic for these boats coming alongside.
We checked again with the keeper the next day and the advice was still the same and we had a second day in York (we did have a reason to be there). With no prospect of the water level dropping sufficiently for two or three days, we then decided to abandon this part of our plans, somewhat disappointed as we have never made it beyond Selby before. On the other hand, we were not at all unhappy with the advice of the keeper was was cautious but eminently sensible and it was, to us, obviously better to be safe than sorry - after all we knew nothing through practical experience of these waters.
From parts that we could see the trip does look as if it would have been worth the effort if the conditions permitted, even to see what it is like coming back into Selby lock. However, we have done Stockwith and Keadby locks so know the factors involved - and how tricky it can get when there is extra fresh around!
Nothing about this experience should deter others from going that way - just be prepared to be flexible if commonsense indicates.