On the intro and outro - where reality is still the chalkboard and you are doing credits and titling - I would toss in your chalk write and erase sounds.
If this seems to generic, try reversing the chalk writing sounds to match your ambient sounds. To go deeper with that idea - If you have a reverb that can provide reverse reverb - try doing a 80 -100% wet reverse that is just the effect bounced and does not include the dry signal.
Turn up your bubbles in the mix when he dissipates - I could barely hear it on small speakers. To add to Hectors comment - to make the splash sound more underwater-y-ish, try a Lo-Pass filter with the cutoff somewhere around the mid-high range to get you started or to get a feel on how cutting highs can make something sound like its coming through a wall or from under-water perspective. Ad some bubbly extras to sell it.
Maybe some underwater swishes for the shark.
Fill it in with some interesting things and you've got a pretty neat artsy piece!
Finish it up!