Five short lines hold a whole feeling of hope with a question mark over it. The speaker remembers watching ships set off "at the beginning of love," when everything seemed to be launching and the future looked wide open. The title, "Rose-colored glasses," suggests that this early view of things was tinted by optimism, so the memory feels a little rosy and a little naive.
Then the poem turns on "but I know not." The speaker can't say whether they will ever see a dolphin. A dolphin is a small, bright, joyful sight at sea, so it seems to stand for the good surprise or reward that love might bring. The speaker has watched the departure but doesn't know if the voyage will deliver anything wonderful.
The mood is quietly wistful. It isn't bitter or despairing, just uncertain, caught between the excitement of beginnings and the unknown of what follows. The ending on a single word, "dolphin," leaves the hope hanging in the air.