Tomatoes are really, really  picky about water. Too much water rots roots; too little stops fruit production. Here you are vulnerable to the weather if it rains a lot.  Cool wet weather generally means your tomato crop is not going to be gangbusters.  If you have dry weather then you can handle the tomato’s water needs quite easily.

 

Tomatoes like a continuous and uniform water supply, but not soggy soil. Try to provide fairly even moisture, rather than allowing the soil to be very dry, then very wet and then very dry again. This stresses the plants, inviting disease and makes them mad and who needs that.

Tomatoes need about an inch of water a week from you and the rain, two inches a week when the summer gets very hot in July and August. A rain gauge helps keep track.  
Ideally, water early in the day, soaking the soil, not the leaves. If you can’t get to it till evening go ahead anyway. Let the water trickle slowly so it penetrates the soil to 8 to 10 inches. During periods of sparse rainfall check soil moisture every 3 or 4 days, more often in windy areas and during the hot summer.

One of the reasons I am always harping on the need for that 2 to 3 inch layer of organic mulch is that a layer of mulch will definitely help slow down evaporation and thus retain soil moisture for the plants.

Porous soaker hose systems hooked up to a mechanical or computerized timer are very effective for delivering water properly to tomatoes as well as the entire vegetable patch.