Boat Wont Start After Sitting?

WE WILL GET IT STARTED PRETTY EASY WHEN FIRST PUTTING IT IN. AFTER WE RUN FOR AWHILE THEN SET AND CHILL AND TRY TO RESTART IT TAKES ABOUT 15MIN TO 1/2 HOUR TO START. POWER STRONG FROM BATTERY JUST TAKES FOR EVER TO FIRE. IF WE SHUT IT OFF IT WILL QUICKLY RESTART BUT JUST AFTER SETTING IT JUST WONT. IT IS A 4 CYL MERCRUISER OUT BOARD. 85 HP. THANKS FOR ANY HELP!!