If tomato plants receive too much water when the fruits are swelling, the taste of the tomatoes is affected or perhaps more accurately ... diluted!
Exactly why this is ... I don't know, but I guess if soil consists of water and nutrients (among other things) the uptake of nutrient levels is reduced when there is too much moisture in the soil because of dilution.
The reason why this has crossed my mind is because of the amount of rain we've had in the UK lately. As long as the temperature doesn't dip too much, tomato plants should be OK. However, cooler temperatures plus wet leaves equals tomato blight ... one of the biggest problems for outdoor growers. Potatoes are also affected by the same disease.