Possible Clues:

• One found at sea taking angle on Polaris, for example
• A swimmer has first to get changed
• As swimmer, sailor beginning to flounder is sinking in silence!
• Celebrity swimmer is well armed in the sea
• Sea creature has first to swim
• Asteroid (in the sea)
• Well-armed customer runs, following one in store
• SpongeBob's pal Patrick, e.g.
• Heavenly marine animal?
• Brilliant swimmer with five arms