Actually, if they’re airtight, the water won’t come out. A little will come out and then none will. The vacuum created by the airtight seal prevents it from escaping. I had this happen once with a 5 gal bucket with its lid completely sealed.
Yup, very important. Just drill a small hole in the lid or near the top of the bucket and that'll solve the issue. Others suggest putting the lid on loosely but a small hole takes the guesswork out of it.