I would recommend a Exped sleeping bag from Bivouac. I just bought one for winter hunting as i sleep cold. The Waterbloc line have plastic welded seams so it is completely water proof on the outside.
Exped Waterbloc 800 Down Sleeping Bag - Sleeping Bags
These are used by the swiss army so have proved their worth. I have the 1000 model which is a -23 rating but would be far too hot in summer.
The other nice thing is that they are made for real sized men. Im a 120kg guy and have incredibly wide shoulders, i fit well in this but anybody can use them with the tightening straps.
here is a good video;
Exped Waterbloc sleeping bags - YouTube
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