We will be buying a train ticket from Charles de Gaulle Airport to Paris (Eiffel Tower).
After the Seine River cruise, we'd like to head off to the Louvre, what transportation do we use?
And from the Louvre, can we purchase train tickets (from anywhere around there) to get back to Charles de Gaulle Airport?
You can buy both tickets from CDG to Paris and Paris to CDG, when you arrive at CDG Airport, from the airport train station ticket office. If you're doing this all within a single day, you can use a Ticket Mobilis day ticket, 1-5 zone. This will cover CDG - Paris / Eiffel / Louvre / Back to CDG.
If you need multiple days of coverage see which Paris Metro Pass to choose.