1985 evinrude 30 hp outboard over revves and and wont go when in water

I just bought a boat with a 1985 evinrude 30 hp which runs pretty good but when I took it out and ran for a while about half speed then throttled up engine revved and boat would barely go. after that anytime I went above idle speed engine will revv and wouldn't go any faster. had to idle back to the dock.after inspecting drive is connected in gear.