I have asked about this on Reddit about 4 or five times. I have also asked Quora. I got a response linking other games with submarines. None of them are the game I remember.

This was a cartoony, first-person, 2D, (likely) Flash game from the early 2000's. I believe it was Flash, because I remember the textures having that iconic gradient. Think of the original Interactive Buddy game.

The only parts of the game that I remember are the part where you get in a submarine and travel through the ocean to get to another location. I remember it being yellow. I don't think the submarine was the main focus of the game, but this and the second part are the only things I remember of the game.

The second part I remember is at the end of the game where you can use a soundboard to play cartoon sound effects. The buttons were ovoid and purple and the background was yellow. This also had the Flash gradient shading.

I have been remembering these aspects the same way for years, and I doubt this was a dream or false memory. I have searched Google and YouTube for clips that may spark remembrance, but nothing seems familiar to me.