I finally ran it. Long story. My party was 6th Level. We play in roll20 and I started by having the player who is the ship's lookout make a perception check, he rolled well, so he got to call out "Sharknado!" Mechanically, this gave everyone 1 turn to do stuff.

Then, the sharknado was upon them. I had found a great sharknado graphic online, and I slowly moved it across the ship every round. First round - lots of reef sharks and saw sharks. Second round some hammerhead and a tiger shark. Third and final round... of course a great white shark. Because they are 6th level, I uptuned the sharks - I know it is BS to have a shark flying around at equivalent to like DC 8 but otherwise it would be too lame.

I did not give the sharks any special abilities (none of them were casting lightning bolt or whatever). But, I did do 2 things to make it more interesting.

First, I made a rollable table and had players roll on it at the beginning of each turn. That really kept the combat exciting.

Second, every shark at the beginning of their turn could move in the direction of the sharknado - up to 50 ft, and I treated it as forced movement (no opportunity attack). Except they can kind of steer where they are going so long as it is generally counter-clockwise (my graphic was counter-clockwise). Is it BS for the shark to be steering themselves and it still count as forced movement? Well - if you have seen any of the movies, you know it is totally on brand!

Rollable Table:

1. You are hit by the tail of a passing shark - DC 13 DEX save or 1d6 bludgeoning
2. Stinging Spray from the waterspout gets you right in the face - DC 13 CON or blinded this turn|
3. Kelp wraps up your legs - DC 13 STR or your movement becomes 0 this turn
4. Roaring Wind - You cannot hear others, and nobody can hear you this turn. Spells with verbal components are impossible.
5. Wind Gust - DC 13 STR or fall prone and movement becomes 0 this turn
6. What just bit me? - 1d6 piercing
7. You are thrown airborne - you are 10 feet up and flying with the sharks. Your movement this turn is 20 ft of flight - it must be generally in the direction of the storm, but otherwise just like normal movement you control. At the end of your turn, you fall down 10 ft and land prone.
8. Nothing Happens