For this session, focus on a consistent output for the swim, 20-sec is not a lot of time to recover so the pace would be fairly close to what you would swim a 1200 in. Then for the finisher you can pick up the pace.

Warm up 

1-2-3 Shoulder CARs each side

3-2-1 Prison CARs

20x Ts Ys and Airplanes

3x10 Banded Capsule CAR

Work

Swim:

12x 100m swim

Rest 20sec between

or

12x 90sec bike erg @ fast pace

rest 30 between

Finish:

3 rounds:

0:30 push ups

0:30 swim/Ski

0:30 Burpee

0:30 swim/Ski