Hello,
there is 3 of options I know of:
– At 21:25 Nissos Kalymnos (the large old ferry) goes from the port of Lakki on Mondays. http://www.anekalymnou.gr/images/dromologia/2018/nkalymnos_01-06-2018-31-10-2018.pdf
– According to the present timetable Anna Express goes from Leros (I think its Agia Marina Port) at 08:05. But they will have a new schedule in a couple of days.
http://www.annaexpress.gr/english.pdf
– Dodecanese seaways goes at 14:30, I THINK also from Agia Marina.
http://www.12ne.gr/en/
Keep in mind that the Leros airport is quite distant from everything else on the island, so option #2 might be a little tight timingwise…
Good luck and enjoy the trip
Ludger