I see you have capability to export. Can you import?
Probably the single biggest reason I would not use an online logbook would difficulty in getting the information in. Even PADI has not been able to come up with a usable online log book. While typing your info into a web form may be acceptable for the diver who does 3 vacation dives per year it is not realistic for me. I want to dump my dives from my computer, add text and photos and have the ability to export these to a nicely formatted PDF logbook page, possibly in a variety of sizes and formats.
Take a look at Subsurface and see if you can offer more features.
The opening page that asks for a dive site and then tells me it doesn’t exist, asking me to add it makes me think this log entry is going to be a tedious process from the start. I didn’t even bother going past that. Don’t make me go one page at a time.