Jumpmaster mode questions

Are you in the Military? 
Otherwise you don't need jumpmaster.

Not in the military, nonetheless I would like to use my Garmin when I skydive. The altimiter is nice and I thought, perhaps, it could give me information about my dive.

What's more, I want to avoid it locking up into dive mode every time I skydive.

I am also a skydiver. You don't need the jumpmaster. It is intended for the ride on the open canopy . Blue skies!

How do you prevent your Garmin from locking up when you skydive?

Do you wear your Garmin? Use the altimiter?

What do you mean by "locking up"?

I wear the garmin watch. Of course, I use an extra altimeter to check the altitude. The watch is too sluggish and not suitable for this.

It switches into dive mode and, I assume it's the pressure, locks in dive mode. I can change the dive screens with the start button, but nothing else. 

This is a known problem that has happened to many.

https://support.garmin.com/en-US/?faq=cJL7KD1StQ6p3LqPBGdWw9

Garmin suggests starting an activity - like jumpmaster - but I haven't gotten that to work. I suspect the power save setting.

I set the power mode for jumpmaster to full power. If the wind does down, I'll be able to try it.

It works just fine as altimeter even in freefall. I could not notice any practical difference compared to classical. The Jumpmaster mode is pretty much useless except for altitude and distance to target. The best is to start it just before jump, otherwise it records airplane track while climbing.
Another issue is you get only QNH altitude, while for skydiving QFE is more practical.
To turn off automatic scuba dive more I turned in Sensors Altitude only.
In video Garmin shows in meters, while classic is in ft. 410ft should be added for QNH for comparison. 

@branimir29

Assuming you start and land at the same DZ, you can calibrate the altimeter to zero in the morning. That way, you get the GFE altitude.