The term "submarine races" was used as a humorous way to convey the message that a couple was or would be going somewhere to park, probably to make out. Here's the thinking. If submarines are underway cloaked in their element, they cannot be seen (whether they are racing or not). So if someone is going to the submarine races, what's this person going to see? Nothing. Here's a hypothetical conversation.
The meaning of the term can also be related to a part of the male genitalia that is often referred to as a "submarine," and the "races" part could be attributed to it's growth during activities such as making out (the faster it grows the better?).
Q: What are you and Lucy doing tonight?
A: We're going out to the lake and watch the submarine races.
The answer communicates that the couple is going to go out and park. That's the long and short of it.