Megaladon Shark available for purchase (50,000 coins) once you max out level 10 of Great White Shark. Megaladon Shark's stats (Bite, Speed, Boost) will be fully-maxed out right after you purchase
See the Mission 6 Map below for the eight shell locations (They said there are more than eight but I won't bother find them all) . Each of the mission requirements must be met in a single run. The use of revives (gems) are ok. Do not use a revive unless you are close to complete a mission.
Megaladon Shark - Mission 6:
1. Crowd Surfer: Eat 100 beach bystanders
2. Jelly Junkie: Eat 100 jellyfish
3. Mine Eater: Eat 125 mines
4. Paraglider Punish: Eat 5 paragliders
5. Harpoon Boat Bite: Eat 5 harpoon boat men
6. Flying Feast: Eat 100 flying fish
7. Boat Breaker: Eat 12 fishing boats
8. 15 Sunken Objects: Find 15 sunken objects
9. Super Mission 6: Score over 5,000,000 points
Mission 6 Map (red is shark spawn location, blue are shell locations)
UPDATE: My take on the 15 sunken objects in one go
You can refer to the list of treasure here from the game publisher
or this map from @QuickMythril (look at the T icons, just forget about the shell locations, since it is a mission 1 map with treasure locations)
I would start from the leftmost with the Guitar, then move to the Jade Dragon, Wakaba, Moon on a Stick.
Be very careful while on Moon on a Stick because the maze is kinda long and any inaccurate move will cost you a life. I played on the phone and look at the map on my PC to determine which way to go. Once out of Moon on a Stick, you would better take on Fake Teeth next since the water current flows from left to right, which makes it easier.
I would then move to Toy Robot, Teddy Bear, Dartboard, Egg Picture, Bog Brush, Robot Gibbon, Leaning Tower, then Lucky Horseshoe. The maze to Lucky Horseshoe is short but quite narrow. I died a couple of times here.
Next stop would be Covenant and finally Ghetto Blaster, which requires you to boost from the shore on to the land.