Never launched from cromer but we have been there a few times without the boat.
The tractors on cromer beach are for launching the fishing boats
East Runton may be a possibility though
Seafront Cromer[slipway] - Boatlaunch
or a little further around the coast Sea Palling where you can definatly launch.
The website appears to be broken but when we were there last year I saw a couple of ribs tractor launched (and loads of jetskis)
Beach Rock Leisure
This explains better though
Seafront Cromer[slipway] - Boatlaunch