The slots are controlled by an EPROM (Erasable Programmable Read Only Memory) chip. The casino would have to pull the EPROM from the machine and insert another one (of the same type of machine) in order to change the payout. The EPROM payout is set by the manufacturer and not by the casino. Swapping EPROMs is something that cannot be done without permission of the Gambling Commission in Las Vegas and is too much work on a cruise ship.
What they will do is move machines from one location to another in order to change how the overall payouts occur. There are better and worse locations for machines.
There are no "better" times to play machines but be aware that cruise ship payout percentages are lower than almost all land based casinos.