Hi Alex!
The only other option getting you there on the same day is via Kos (assuming you miss the 15:00 ferry). The problem is that it takes almost ALL day. You’d have to:
1. Go to Rodos port
2. Take “Blue Star 1” to Kos at 17:00
3. Arrive @ Kos at 20:05
4. Take a taxi or bus to ‘Mastichari’ port (~30km)
5. Take “Kalymnos Dolphin” to Kalymnos at 22:30
6. Finally arrive at Kalymnos / Pothia port around 22:50
Another suggestion, so you don’t spend the entire 19th on the road, is to stay overnight at Rodos Town (there are tons of things to do – except climbing…) and take “Pride Dodekanisos” from Rodos to Kalymnos the next morning. It leaves Rodos @ 08:30 and arrives at Kalymos @ 11:50. Just an idea.
I hope this is helpful!
Katie