I agree with everyone else that a watery stuffed pizza can result from (1) use of straight, fresh, normal mozzarella instead of low moisture mozzarella, (2) a watery sauce, (3) 'wet' (i.e., not sauteed) veggies -- particularly peppers and (4) not letting the pie 'set' a few minutes after it's done.
I've found that searing diced peppers (or whatever wate-rich veggie I'm using) on a very hot pan (slight or no oil added, maybe a shake of salt) gets rid of enough water to avoid swampy stuffed pizza. I only sear enough to very slightly darken the edges of the veggie -- be it peppers, onions, or sometimes even zucchini. Real hot pan, constantly moving the veggies around so they don't burn, but just a few minues -- five minutes on hot, that's about it.
If you use frozen spinach, be sure to squeeze out the water. Fresh and dry spinach, in my view, is probably preferable to frozen, tough I have used frozen chopped spinach plenty and squeezed the water out, with good results.
Good suggestions so far, you all.