You will certainly enjoy having shorts to wear during the day. Evenings can cool a bit, so slacks may be more comfortable, but it will also depend on when in Oct you will travel. The temps are still fairly warm throughout the month and the ocean is still warm enough to swim.
I'd take both, as a cool spell can always occur- especially late in the month.
Of ocurse, if you only pack shorts and find you will need something warmer, there are lots of outlet stores available to find just what you need!
The average high in October is 77°, so shorts will be OK for the most part unless you get hit with some unseasonably cool weather. Night time should probably be pants because the wind can make it feel a little cooler.