Here are some facts: The Core is Water Resistant to 30m/100feet. This is a conservative number from Suunto. If the Core was not meant to be swam/snorkel with, it wouldn't have a dive profile.
Dive Profile: This is a mode similar to the "Altitude" and "Barometer" profile except it is designed to take measurements under water. This essentially turns the Core into a Dive computer for dives up to 10-12meters in depths. (Some owners have taken theirs deeper)
While in the Dive profile as seen in above pic, the top line displays the deepest measurement while in the dive mode. If you switch out of dive profile and go back to dive profile, the number will reset to zero. The second line displays the current real time depth. This moves up and down as you dive.
The third line displays the water temp. It isnt effected by your body temp as much as it typically would be in barometer/altimeter profile.
You may also press the Cores buttons under water. This is typically avoided in other watches but the Core uses under water button. In reality, the Suunto Core is a ABC and Dive watch unlike any of its rivals.
DIVE LOG RECORDING:
You may also want to start a Dive log while you are swimming. This works similar to a Log in altimeter profile except the record interval is always going to measure every 1 second regardless what is set to record at in the "record interval" section of the memory. This will give you 30 minutes of record time per log. So you could record 10 separate logs at 30 minutes each log.
Note the bottom pic: This is a 2minute24second log I took while swimming in a 8 foot pool. It works very similar to the altimeter log when reading it back including the real time moving graph of your dive....very cool.
As typical Suunto excellence, I found the accuracy of the measurements to dead on. At 12" of water the Core displays 1 foot.
DONT BE AFRAID TO TAKE YOUR CORE DIVING.