Hey Oscar, I know what you are talking about, especially the watermelons although with research and genetically modified seeds, they are producing sweeter and tastier fruit without the seeds.
An example of a fruit that tastes the same without seeds is the Tango Tangerine. It is basically a seedless version of W. Murcott. It was produced by irridatiating W. Murcott causing a mutation that likely renders the pollen inactive.
I really like the Sweetheart Lychee for the extremely large, golfball sized fruit and the shrivelled seed. Some other examples of fruit that tastes good if not better with atrophied seeds are Durian and mango. From my personal experiences, I have purchased several Durians with atrophied seeds and they tasted much sweeter and creamier than their large seeded counterparts. Also, for Manilla type mangoes(and probably others), I always try to select the Thinner fruit as they often have atrophied, very very thin seeds compared to the larger wider fruits with the fully formed seeds. My theory is that the energy that would have gone into producing the seed went into the formation of more sugars. This is just what I have noticed and I could be totally wrong