Just to be clear, there are two Hohner harps that carry the name "Blues Harp." The older model, long-discontinued, had a comb made from the same wood as that used in Marine Bands, and the comb would swell if you soaked it. You can easily recognise these harps as they are nailed, not screwed, together, and they don't have "MS" (modular system) stamped on the cover plate. The modern Blues Harp has a comb made from wood which will swell hardly at all. The harp is screwed together and has MS stamped on it. The parts of these modern harps are totally interchangeable with any other harp with MS stamped on it.

If you soak a Marine Band or an old-style blues harp the swelling will have an unpredictable effect on the shape of the comb partitions and may loosen the nails that hold the plates to the comb. Soaking won't ruin the reeds as such but may hasten corrosion of the rivets that hold the reeds to the plates and of the inside of the cover plates. I know what the traditionalists say about Marine Bands but my advice, for what it's worth, is to just avoid these outmoded harps with their crude construction. There are plenty of better alternatives around these days. After a long night's playing I wash out all my diatonic harps under a stream of tepid water and shake/tap them out vigorously, then leave them out to dry overnight. This prevents that horrible crud from building up in the mouthpiece holes and keeps the reeds sounding bright and responsive. The brief wetting doesn't seem to affect the harps adversely in any way, even the wood-combed ones.