If you can get a wig from a library, that's your best short term option as you don't want to waste money on a wig you've bought in a rush. I wore a wig right through treatment and those who didn't need to know, didn't. Mine was synthetic, looked great, easy to wash, no styling required, cost about $200.
But it's hard to know how you will feel until you actually start chemo. Do you know the people you will be sharing a cabin with? Most people start losing their hair 14 days after chemo so you may be fine on the cruise from that angle. You are advised to take care about temperature levels and possible infections but many people are fine. It might help to calmly tot up the pros and cons of taking the trip and then decide. Best wishes. There is no good time to start all this but you have started and therefore can start thinking now about when treatment ends.