Slipway below the bridge is run by Humber rescue. They ask for a donation to use it. If I remember rightly £5 for a tractor launch.
It's a concreate slip onto a shingle beach and the tractor is definately worth it especially at low tide.
I went out on a Sunday and the boat house was manned all day if your looking at going during the week I would give them a ring or drop them a mail to check access.
They were really good, nice easy launch and once they'd noticed I was ready to recover they were straight out to pick up. The tide runs a reasonable race and at the time was perpendicular to the wind which made it interesting but I'm sure you'll have no problems.