Eiffel Tower--they have a new system, and tickets seem to be released 45-60 days ahead, depending on what time of the month your visit lands. For example, they will be releasing tickets for all of August the middle of June. Buy from the official Eiffel Tower website.
Catacombs--if you want to buy the skip the line ticket, buy it as soon as you know what date you want to go from their official website.
Versailles--never sells out, but you do want to buy tickets before you go so you don't have to wait in the ticket line (the security line is long enough!). You can now buy a skip the line ticket from the official Versailles website that is supposed to get you in within 30 minutes of your ticket time. And you can buy a Kings Apartment tour for an extra 10€ that lets you skip the lines entirely (book that tour as soon as you know your dates, because those do sell out).
Boat ride--if you just want a 1 hour cruise with no meal, you don't need to book ahead. Just walk up whenever you decide to go. If you want a dinner cruise, then you must book a month or so ahead.
And just a few additional points to Annette's post.
Eiffel Tower sells out advance tickets very quickly, so if you don't happen to get them on the release date, you may not find them available except through 3rd party resellers at a premium price.
The Catacombs used to be that way as well, but since they changed the price structure and charge a premium for online/reserve time ticket, they don't seem to be selling out as quickly. Nice thing about catacombs, you don't have to be concern about choosing date/time with bad weather, like for the Eiffel Tower.
Seine boat tours -- the only reason to buy in advance online would be to save a few €'s, as they may have on online deal. For example, Vedettes du Pont Neuf offers €2 off online tickets (anytime on a specific date) or €4 off, if you pick a specific date and time.Edited: 11:43 am, June 07, 2018
Thanks Annette C and Jamielobo for your replies and very helpful information. So I reckon I’ll get the KAT tickets together with my TGV tickets where possible, then book the rest accordingly. I was thinking about a dinner cruise initially but had since decided against it as I have a feeling I’ll be very distracted by the views and totally forgot what I will be served, doing no justice to the Chef at all 🤔😅
Thank you both for your help!