Solid opals can be cleaned with a damp cloth with a bit of detergent. It can be submerged in to water with out a problem. Because an opal is a bit of a soft gemstone, there is a chance that it can get a few scratches over time. If that happens it can be polished back up again with out a problem.
Triplets and doublets.
Triplets and doublets shouldn't be submerged in to water because there is a change that it can get in between the layers where the cement is. Because they are cemented together they are a bit more fragile than a solid opal, but if the opal has been set right, there shouldn't be a problem with that. If you take a doublet or a triplet opal to a jeweller to be set in to a piece of jewellery, make sure that it is a experienced jeweller that does the job. the biggest change of "chipping" an opal is when it is set.