A CONSUMMATE CRUISER SPEAKS   Here's a guy who sailed in multihulls as an infant, and now owns DELPHYS, a 34' Searunner trimaran which has been called, by John Marples (one of her designers), "The most developed Searunner in the world."  And her usage? Well, with her owner/builder crew of Mark Johnson and his wife Mariam, the boat has visited  some twenty countries.   To learn what Mark would do -- if indeed he had it all to do over again -- listen-on!