It means dipping (swimming or having a bath) in the foamy hot tub. As you know, hot tubs look like pots and they are "hot" as a pot when you boil water in it. It's also a reference to "swimming in grotto" line. And the water is blue, obviously. :)

"to dip" - often used to refer to going for a swim or bathing in the ocean..."it was such a hot day we decided to take a dip to cool off" - pot - perhaps the grotto looks like a pot - blue foam - perhaps the water is bubbly and dark blue Remember this is a song and like all other art forms does not have to follow "grammatical rules"

It's not really clear what they are talking about; it doesn't make too much sense. From the context, they might be talking about a bubble bath (the soap can make the bubbles light blue).