I just typed into Google.com
Dominican Republic Puerto Rico ferry
and here is the URL for the schedule and price for you:
http://www.puertoricowow.com/html/ferry.html
FYI they say it is 11-12 hours ( it is at least 80+ miles) so I am not sure a round trip in a day won't be essentially a round trip that literally takes 24 hours.
You can stay on the DR/Mayaquez ferry and it leaves at 8 p.m. arrives 8.am. in
PR leaves immediately at 8 a.m. and arrives back in DR at 8.pm.
For $109-$247 each way per person you might want to get off and stay a day at one end or the other. There is another non car ferry that takes 14 hours (!)