Swimmer ultimately floundering: where is that wretched rescue vessel? (5,5,5)
I believe the answer is:
great white shark
'swimmer' is the definition.
(great white shark is a kind of aquatic vertebrate)
'ultimately floundering where is that wretched rescue vessel?' is the wordplay.
'ultimately' suggests the final letters.
'wretched' is an anagram indicator.
'rescue vessel?' becomes 'ark' (I've seen this before).
The last letter of 'floundering' is 'g'.
'where'+'is'+'that'='whereisthat'
'whereisthat' anagrammed gives 'reatwhitesh'.
'g'+'reatwhitesh'+'ark'='GREAT WHITE SHARK'
(Another definition for great white shark that I've seen is " Fish".)