The mermaids weren't until S9, the last season. We still haven't gotten through the heart of the period when the show was at its peak popularity.
I looked at the show's ratings to refresh my memory as to the popularity
of Love Boat, and Seasons 4 and 6 were the highest rated of the run, in the
top 10 shows on TV.
season 1 (1977-78) Neilsen rank 13th
season 2 (1978-79) Neilsen rank 16th
season 3 (1979-80) not in the top 20
season 4 (1980-81) Neilsen rank 4th
season 5 (1981-82) Neilsen rank 15th
season 6 (1982-83) Neilsen rank 9th
season 7 (1983-84) Neilsen rank 17th
season 8 (1984-85) not in the top 20
season 9 (1985-86) not in the top 20