GeezerCouple

This can vary with the cruise line (and here I'm including working with a TA who books it all directly through the cruise line also, vs. getting the air separately).

There can be a separate fee to request a specific date/time, rather than waiting to see what you've been assigned, including the possibility of a couple of connections, long layovers, and also arrivals not all that far in advance of the ship's boarding time.
That might mean a fee for the service as well as extra charges for the higher fares.

Once, about 5 years ago, we asked our TA to ask the cruise line "how much to get <this particular flight on this particular date in this particular class (J)> and when we found out that it was indeed less than we'd have to pay if we booked it ourselves, we told her we'd take it. We were then also able to select our seats, pretty much as if we'd done the booking ourselves.
Because we've *never* allowed the cruise line to "select our flights for us" (!), I'm not particularly familiar with how seat selection/etc., would work under those conditions.

But bottom line, there is NO way that we are willing to arrive at, say, 10am and then sail that afternoon. Too many chances for a "miss". I do not think all cruise lines actually promise to get you to the next port if there is a "miss", but I'm not 100% sure. But even if they did, we don't want to be flying around chasing the ship (especially if there are major flight disruptions), when we could have been on the ship from the start.
So we always fly in at least one day in advance, and often two. If it's a new place that we want to enjoy, we'll plan a mini-vacation prior to sailing, which then also provides a very nice buffer.

GC