As you realize we cannot predict weather. Over the last two years we have had both extremes. Prepare for the worst and hope for the best.
If you want to check the weather conditions in real time you can find the info here. Note: the Utö water temperature will be similar to the Sandhamn water temperature.
Like everywhere we can have wind, rain and un-seasonably cold. Make sure you bring clothes to handle this.
Salt water. The water in the Baltic has very low salinity. Some teams will even race without swim goggles.