Let's start with what a 'typical' (as in 'non-mutant') Overt Dolphin Moreau:
Dolphin Moreau are a blend of human and dolphin, typically bottlenose. [...]
Unlike other Moreau, Covert Dolphin Moreau have little to no body hair.
Overt Dolphin Moreau have distinctive skin patterns, with a dark blue or black back, a white front and golden stripes down their arms, ribs and legs. They have pronounced webbing between their fingers. [They] have tiny external ears and unlike other Moreau Overt Dolphins tend to be hairless.
- Overt: +2 strength, -2 dexterity, +4 intelligence, -4 wisdom.
- Medium: As Medium creatures, Dolphin Moreau have no special bonuses or penalties due to their size.
- Dolphin Moreau base land speed is 30 feet.
- Dolphin Moreau base swim speed is 30 feet.
- Hold Breath: All Dolphin Moreau can hold their breath for a number of Minutes equal to their constitution score.
- Low-Light Vision: All Dolphin Moreau can see twice as far as a human in starlight, moonlight, torchlight, and similar conditions of poor illumination. They retain the ability to distinguish color and detail under these conditions.
- Blind Sight: Overt Dolphin Moreau have blind sight with a range of 30 feet on land and 60 feet in water.
- All Dolphin Moreau get a +2 to swim checks and add Swim to their list of Class Skills.
- Automatic Languages: Common and Aquan.
- Favored Class: Mage
- Avoided Class: None
(All emphasis mine)
This already gives your character a very distinct look - I think you don't need any of the cosmetic mutations for eyes, skin and hair - as your character will already posses no hair, and have skin and eyes matching the norm for a Dolphin Moreau. The Fins and unnatural voice cosmetic mutation seems very fitting, on the other hand.
Moreover, you'll note that you already get blindsense (60 feet in water, 30 outside of them) - which is very like the Echolocator mutation (blindsense 60 feet everywhere) - so, unless you'd like the improved range outside of water, you don't need that.
Looking at some of the other mutations listed in the question, I'm not really sure how they make the character more 'dolphiny' - some of them even seem to contradict the dolphin concept - let's go over them briefly:
Thin Skin and Heat/Cold Susceptibility - from Wikipedia:
All dolphins have a thick layer of blubber, thickness varying on climate. This blubber can help with buoyancy, protection to some extent as predators would have a hard time getting through a thick layer of fat, and energy for leaner times; the primary usage for blubber is insulation from the harsh climate.
So, you'd expect them to be more resistant to extreme temperatures and damage, not less so. Especially if you want your character to be comfortable with diving deep - where the water are colder.
Light Sensitivity - Dolphins are active during the day, and don't have any problems with bright light...
- Gills - Dolphins can't really breath water, they just hold their breath for very long (which the Moreau template already has), you may want to increase your Con rating, or work with your GM on a weaker mutation to allow you to operate longer under water (maybe "Hold Breath" for 1 MP and 6 times the duration - like the dolphin from Pathfinder D20?).
Other mutations which may be relevant are Enlarged Form (6 MP), Tail (1 MP), and possibly Pheromone Repulsion (-1 MP) to represent the 'fishy smell' outside water.