Great White Shark available for purchase (35,000 coins) once you max out level 10 of Tiger Shark.
See the Mission 5 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.
Great White Shark - Mission 5:
1. Great White Attack: Eat an enemy great white
2. Over Fishing: Eat 500 fish
3. Mine Sweeper Pro: Clear 15 mines
4. Subzero: Destroy 3 submarines
5. Survival Master: Survive for 15 minutes
6. Big Gulp: Eat 10 gulpers
7. Speed Eat: Eat 100 things within a minute
8. 8 Sunken Objects: Find 8 sunken objects
9. Super Mission 5: Score over 1,000,000 points.
Mission 5 Map with treasure location (red is shark spawn location, blue are shell locations) - courtesy of @QuickMythril. I was lazy to edit the original map with only shell locations