Here are the rules on underwater movement:
While climbing or swimming, each foot of movement costs 1 extra foot (2 extra feet in difficult terrain), unless a creature has a climbing or swimming speed.
I have a question related to the highlighted passage: if a creature has a swimming speed, but chooses not to use it, would their movement be penalized as per the above rules, or would they get the same benefit [no speed reduction] that creatures using their swimming speed have?
One case where this would be useful is if a character has a walking speed of 50 feet and a swim speed of 30 feet. Since the character has a swim speed, can it use its walking speed to move faster (since it no longer has to spend extra movement)?