Possible Clues:

• Fellow relaxed but not as sea creature appears
• Slow swimmer to take up position at opening of a hole
• Sea denizen manta's put in a tizzy with ease, it's said
• Ocean mammal
• Round-tailed aquatic mammal
• Plant-eating aquatic mammal
• Dugong's cousin
• Beast or man where there's driving
• Sea creature spread out, with head tucked behind tail
• Aquatic creature providing the human race with a starting-point