how strong is pvc pipe cement? for example, can it seal lids on & keep them water tight if they are to be used in building a raft/boat.?

Five gallon buckets are typically HDPE which doesn't glue very well without an adhesive specifically designed for it or some changes to the surface structure of the plastic. The glues used for PVC actually react with the polyvinylchloride to form a seal. Given that high-density polyethylene is a totally different plastic, PVC won't work.

The problem is that the material used in your buckets probably isn't PVC. Two, if you ever look at a PVC connection, they are a fairly tight tolerance, and a lot of surface area to glue.

So your problem isn't the strength of PVC glue, but what you're trying to use it on.

Home water pressure can be +150 PSI.