Its clear blue water and no sea weed, if sea weed gets wahsed up after a storm it will be cleared away, in august sea temps around 81f. As for jelly fish it will just be case if they are problem this year or not. I have been to majorca 12 times and its only been the last few years there has been a problem, I was in majorca 2007 and they weren't a problem. Jelly fish are not native to the islands and move into the area from the eastern med if there hasn't been much fresh water run off into ther med from where snow has melted the mountains in spain and france making the water slightly warmer and saltier then normal.

The sea will be chilly in May not warmed up much and the weather is a bit hit and miss as well so early in the season.

Here is May for last year and you can look back for 7 years + by changing the year. The temps are taken at palma airport.

http://www.wunderground.com/history/airport/LEPA/2008/5/27/MonthlyHistory.html