### Clue: Love, constant, above all" — Boatman's sea creatures (6)
### Answer: OCTOPI 
 ### Explanation: Definition: sea creatures
 O(letter representing 0;"love" in tennis scores) + C(symbol representing a quantity in mathematics or physics whose value does not change, or, specifically, the speed of light, a universal constant) + TOP(above all, as in "top of the class") + I(first person pronoun for the setter, Boatman, used self-referentially).
Defn: … whose singular is "octopus".