It says you get a swim speed but does not mention your walking speed.  Do you retain it?

If that is true, it is slightly better than a potion of gaseous form, (with the exception that you have no fly speed).  If it is not true, then it is useless outside of a water environment, but great in one.

The wording is definitely a bit ambiguous, but in my opinion you retain your walking speed based on:

• You clearly retain most of your statistics
• The potion never says swimming is your only method of movement, unlike gaseous form.
• Using a walking speed isn't in the Limitations section.
• The bit about extinguishing flames also kinda implies you can be on land (I know you can have oil-based fires on water, but still.)