The reviews below are a bit mixed - some very good, some not so great. So I'll try to give some balance.
We stayed for one night in the Captain's Room. It was spacious, had a big comfy bed, sofa and chair - and a stand alone bath. It also overlooks the river and where we parked our car. We had a good sleep (perhaps reading in the hot bath is a good sleep aid!). The bathroom was good too, nice shower. In the evening, we drank in the bar and then ate dinner in the restaurant - super beef & ale pie with veg followed by syrup sponge and custard. Good service! And the location is great - old KL was surprisingly nice and we had a walk to the church before dinner and a walk to the market square before breakfast!
What is perhaps letting the hotel down is attention to detail. A couple of things in the room and also at the breakfast bar could have been better but I suspect they just weren't spotted. So my rating is really 4 minus with the great location, nice room, good dinner and nice cooked breakfast let down by some little things.
However, on balance, we had a good stay. So, if you can live with it not being perfect, then the captain's room and the restaurant make for a nice stay in a very good location for seeing old Kings Lynn.