Air getting trapped in the suit isn't going to make you float head down or anything like that. You'll find that when you're in the warmer air your suit may need to be bled - just pull the neck seal away from your neck a little and squat down a bit - the air will rush out. When you're in the water, that air left in your suit is going to contract from cooling, and you may find that the suit is now tight against you. That may feel a bit cold and give the idea the suit is leaking, but it's not.

If you're suit leaks in the water and fills up, you're not going to sink. Your suit offers no real bouyancy, and filling with water will just keep you neutral in that respect. You'll be wet. And when you go to get out of the water the water weight in your suit will make things tough if there's enough of it.

I think the biggest safety thing for a dry suit is to make sure you dress appropriately under it. The suit itself offers almost no insulation, so you must consider the water temperature if you fall in. What may be comfortable out of the water may not be warm enough in the water. Wear enough layers to keep you warm for the temp of the water.

PFD on outside of dry suit.