Just got off the Volendam last Monday and had a great time. It rained in every port but it didn't stop us from doing anything. We took rain gear (ponchos) and I only wore a light jacket with shorts anyway. I would say that the average temp. in each port for us was in the low 50's but that may change for you in August (take bug spray too). The only problem we encountered was a cancelled shore excursion due to bad weather so we shopped instead. Service was good, food was great but the crew needs to learn to smile more - they take their work way too seriously. Also, the average age was way down - there was 70+ kids on board and Club HAL had them very busy. You'll have a great time and if you have any specific questions, send me an email.
Scott