Three friends and I want to do Stockholm -> Tallinn -> Helsinki -> Stockholm for a three-day weekend in January (leaving on the 13th, coming back on the 16th). The original plan is :

* Stockholm -> Tallinn on a **Tallink-Silja Line** ferry
* Tallinn -> Helsinki also on a **Tallink-Silja Line** ferry
* Helsinki -> Stockholm by plane.

Now the thing is that Tallink-Silja Line offers *day cruises* (leaving Stockholm on the 13th, arriving in Tallinn on the 14th during the morning and leave on the 14th in the evening back to Stockholm) whose tickets are much cheaper than standard one-way tickets. For four people and the dates I provided above, a day cruise is 78€ and a regular one-way ticket is 159€ (same as a return ticket that's not on a day cruise, me thinks).

Of course, I am fully aware that we will lose the return leg of the day cruise if we choose to book the day cruise and only do the Stockholm -> Tallinn leg, and it's not a problem at all. I'm simply trying to find out if there is more to it. The Terms of Use don't really mention the issue.

**Could there be consequences in buying the cheap return day cruise tickets and not taking the return leg of the journey?** 

I know this question has been asked multiple times on TSE and with different carriers, but please take into consideration the fact that I also want to use Tallink - Silja Line for the second leg of the journey (Tallinn -> Helsinki): I wouldn't want to be on some sort of a blacklist or anything.