Buoyancy Proficiency

[SlugLife]

Whenever I get in the water, boat or shore dive, my body naturally goes horizontal.

Try crossing your feet to ensure your not unknowingly kicking. That aside, I'm leaning towards an equipment, weight, buoyancy, and trim being the primary issue.

Others mentioned the BCD is back-inflate, so you have positive-buoyancy behind you. Next, do you have weights in those front/side pockets? Those weights are negatively buoyant.

This setup will naturally cause you to pivot forward, until the air is up, and weights are down. If you can move some of your weights towards your back, this should help you remain vertical at the surface.
At your current experience level, you definitely want ditchable-weights, so don't move all your weights to the back, just a few.

Unfortunately, there's probably not much you can do to balance out the air-distribution, other than ensuring your harness straps are appropriately snug, or perhaps not COMPLETELY filling your BCD. The important thing is to have enough air on the surface your head is comfortably and safely above the water (and perhaps waves), but if you're not over-weighted, it doesn't have to be completely full.

If there are no pockets in the back, a weight-belt is another ditchable option, and you just put weights in the back of the belt. There are also "trim weight pockets" you can attach to the straps of the scuba-tank, but these are not ditchable.

 

[BlueTrin]

To add to @SlugMug excellent explanation and drawing, the reason why inflating a tiny bit less will help a lot with this, is that the air in the pocket will always move up, so when you deflate from the position of the drawing, the empty part will be at the bottom, in the section under water in the drawing.

 

[SlugLife]

Whenever I get in the water, boat or shore dive, my body naturally goes horizontal.

To add to @SlugMug excellent explanation and drawing, the reason why inflating a tiny bit less will help a lot with this, is that the air in the pocket will always move up, so when you deflate from the position of the drawing, the empty part will be at the bottom, in the section under water in the drawing.

Adding to BlueTrin's comment:

I updated the diagram to show what a more balanced setup may look like. (There are other variations, but they're not relevant this early in OPs dive-career). Like a "center of mass" you can think of a "center of bouyancy", being the orange dot in the diagram. You ideally want to be as balanced/symmetrical as practical, front/back, left/right, top/bottom around that point. As mentioned, not much you can do about your BCD location. However, the diver is also slightly buoyant and by repositioning the weights, and having a little less air in the BCD, we can mostly eliminate the physics pushing you to tilt-forwards on the surface.

Don't worry about being "perfectly vertical" on the surface, you just need that good-enough that you're comfortable and "acceptable." Far more important is having a neutral/balanced buoyancy underwater, where you are diving horizontal. The same principles apply here. I'm getting slightly ahead here, but you can re-use these same principles to fix "tirm" problems underwater. If you start tilting in one direction, you may need to reduce positive/negative buoyancy, in order to be more balanced around that center of buoyancy.

 

[Aviyes]

Final piece is just learning to trust that you will have air to breathe as you descend. You are fighting natural instinct to have air in your lungs when you begin descending and as such do not breathe out as much as you think. This will change as you become more comfortable and are able to exhale more of your lung volume comfortably when starting your descent.

I had this issue when i was starting out after certification. At the start of a dive trip I would overweight myself by 2 lbs for the first dive or 2. By then I would regain my comfort in the water and could ditch the 2 lbs. I called it my "anxiety weight". No longer a problem.

Glad you have been having fun with your new certification and the diving addiction it brings with it!!