I use a foam brush and a good one with the fine foam not one from the dollar store.

once you puddle on some varnish, do an area about a few square feet at a time. Go one way than the other so you get an even coverage. Then you have to tip it off. You lightly drag the brush over the surface leveling any brush strokes - go with the length of the boat. You do this with a bristle brush or foam brush - whatever you're using. If the varnish is too thick, it will not level well within the time period considering the heat. In your hot condition, varnish in the morning. Dry spots are caused by not going back and forth with different angles when you first apply it (cross hatch). Doing that insures you have a fairly even coverage. As soon as one section is done you move on to the next section keeping a wet edge from one area to the next.

the entire deck should take no more than 20 minutes to varnish - or maybe less.