London to Paris can be done by direct Eurostar train fery fast (but with a railpass you need to reserve this one very early).
Going to Switzerland from Paris, there are many ways (depending on where you’d like to go); so this one is no problem at all, too. But you do need to reserve high speed trains in France, too.
It’s also no problem to get to your cruise without flying (I assume you board in Civitavecchia?). If yes: I did some cruises from there and the station is also near the cruise port. I’d go there directly and visit Rome on your way back. Especially here there are some nice ways to go, too (e.g. through the mountains or partly by boat).
Roma to Venezia is a frequent connection; no problem to go by train at all.
I’d recommend a 1st class railpass for this as there are a few long train journeys and it’s not that much more expensive.
If you’d like to get any concrete suggestions just ask. But then it would be of course important to know exact dates and number of persons.
@Hektor Thank you for your help!
How do I need to book the Eurostar tickets? Direct on that website or will the rail pass?
Also are we trying to do to much in too short of a time frame? Travel dates are Sept 11th -23rd. The cruise is Sept 15- 22nd.
How do I need to book the Eurostar tickets? Direct on that website or will the rail pass?
Your pass is your ticket.
For some trains, such as Eurostar, you also need a reservation. Where best to book reservations depends. Here's an overview:
https://www.seat61.com/interrail-and-eurail-reservations.htm
Eurostar reservations can be booked without extra booking fees on www.raileurope.com (desktop website only).
It's a bit much in my opinion yes, you'll spend more time travelling than in the cities !
Honestly I'd only visit Rome before the cruise. This city is worth a few days for sure and September is a good time : still hot but the worst crowds are gone after peak season.
Try to find return flights from Rome in that case.
But maybe you could fit a few extra vacation days to go to Florence or Venice...
You can find more information about traveling by train in Europe here. It's not just about routes and schedules but also includes information about sightseeing:
https://rail.cc/
Thank you everyone! Please share any tips and suggestions you may have for us 
