Del Monroe is probably best known as Seaman Kowalski in Voyage to the Bottom of the Sea (1961) and the subsequent TV series based on the film, Voyage to the Bottom of the Sea (1964), He was thrilled to play a seaman in the big-budget film, but one day on the set he noticed what he believed to be a silly-looking mistake. He pointed it out to producer Irwin Allen, not knowing if Allen would take offense and end Del's career right there. It turned out that Allen didn't take offense at all, and in fact hired Monroe to play the same role in the series, which lasted for four seasons (1964-68).
Monroe has stated that he enjoyed doing the "Voyage" series, no matter what the plot of the episodes he was in--Cold War spies, deadly amphibians, monsters from outer space--he loved them all. He didn't even mind the change in the show's premise from an adult sci-fi series in the first year to a more children-oriented viewpoint in subsequent seasons.
In the third season he had a meaty part in the episode "Deadly Waters", in which his character had to deal with the problems of his troubled brother. The next year saw Del's character as the focal point of another episode, "The Deadly Amphibian", in which he was taken over by a deadly undersea creature. However, in the final season he was given another chance to strut his stuff in "The Return of Blackbeard", in which he was required to behave like a rollicking, devil-may-care pirate, but the episode did not go over well with series viewers.
Irwin Allen also tapped Del for appearances in his other TV series, and Del showed up in an episode of The Time Tunnel (1966), but turned down a chance for a recurring role in Allen's Land of the Giants (1968).
After "Voyage", Del did guest roles such TV series as Wonder Woman (1975). He found time to return to the big screen when he secured a part in Speedway Junky (1999), showing up in the beginning of the film as a grizzled old-timer. In 2004 a few "Voyage" cast members got together (not including Richard Basehart, who had died) for a convention and Del was speechless that so many fans turned out to celebrate a 40-year-old TV series.