Amy, in searching around I found that most yellow watermelons have black or blackish grey seeds.* The orange ones sometimes have light colored seeds.* That's why I asked if you had grown these from seed or purchased plants.* Here's some info on the different colored ones.
http://www.seedfest.co.uk/seeds/wate...termelons.html
I think the taste is because it isn't fully ripe.
Newt
When weeding, the best way to make sure you are removing a weed and not a valuable plant is to pull on it. If it comes out of the ground easily, it is a valuable plant.