Q:

What does "do sea on classes" mean?

I see this sentence on https://twitter.com/jfbastien/status/1003697787911966720 

A: I've made my submissions for @cpponsea, have you?
B: Sorry, I only do Sea on Classes 
C: you can do classes on the sea at C++ on Sea - and in class, too

Google tells me nothing: do sea do sea on class

A:

Its a joke, a pun based on the fact that C (the programming language) and "sea" sound the same. 
There is going to be a conference C++ on Sea, in a coastal town (some coastal towns have the suffix -on-sea, eg Southend-on-sea). The pun is then continued, as "on the sea" means in a boat; C++ has classes but C does not. 
So if you were programming in C++, in a classroom on a boat, while attending the conference you could be said to be doing "classes on the sea at C++ on Sea - and in class".

