best info regarding restaurants in cherbourg or anywhere in france, is in a red Michelin guide. There are 4 grades of food: listed (no star) and then starred 1 to 3. The ones with the smily micheln man indicate specially good value.
As far as i remember (aint got the book here) the Cafe de Paris is listed and was very decent.
There are plenty along the front moules fritte the lot. Most pretty reasonable. Cross the road from marina walk through the narrow side road past the church you will pass a dance school. Walk up to the corner past the Hotels and there is one on the corner very nice steaks moules etc reasonably priced.Or cross over the road from here and there are about 10 to choose from.
The cafe de paris used to be great in the 1980s. It was like being in that Renoir, 'bar at the folies bergeres' The food still is good-excellent but unfortunately the decor has been tarted up with the sort of mauve velour the French love so much. Ah well...