A banana split made in a butt crack
I went to DQ and they wouldn't let me order a banana brown boat
Being under the influence of a depressant (IE Alcohol) and needing to quickly pick yourself via a stimulant (IE Cocaine) up for a particular reason.
When Capt Witt in Flight needed to appear for a NTSB board hearing after a massive hangover he used a banana boat to get himself ready.
When you and your friend both have a boner at the same time.
Friend 1: Yo, I'm hard
Friend 2: I'm in the same boat, man
Friend 1: A banana boat...
When you use a peeled (for natural lubrication) banana as a dildo.
I had the best banana boat of my life last night