^^^ I'll stick to rubbing two, er, sticks together!
For anyone who doesn't know: Sea Lanterns!
At their fundamental level, Sea Lanterns are in the same league as Glowstone. If you break them without a Silk Touch pick, they collapse into particles of Prismarine Crystal. It requires 5 crystals and
4 Prismarine Shards* to craft a Sea Lantern block.
So, in absence of a Silk Touch Pick, one Sea Lantern requires five Prismarine Crystals and four Prismarine Shards. If you have trouble with math, that translates into mining a minimum of two existing Sea Lanterns (or getting lucky with five Guardians) and the ruthless slaughter of anywhere from two to every
Guardian around an Ocean Monument.
For those of you looking to harvest Sea Lanterns for a living, I strongly suggest you go with your best armor. You will
need a fancy helmet with Respiration (preferably III) and Aqua Afinity. I also suggest a Silk Touch pick so you don't waste effort and energy. Frontal attacks are not the best way in. Once you locate a Monument**, tunnel in at a distance and work from underground into the Monument, then work from the inside out. You will have less chance of getting surrounded.
One more thing: There will be as many as 3 Elder Guardians*** (bosses) which induce mining fatigue from a long distance. This is apparently to prevent you from mining the Monument until it has been rid of the yuckies. The payoff is three new types of greenish block and (most importantly) Sea Lanterns!! The treasure is basically worthless (8 gold blocks—the sale of all 8 might yield 1 Lantern once they get into the store) and not worth the effort IMO. New blocks and Lanterns are the real catch in these things.
Due to the VERY low obtainability of these rascals, I suspect Sea Lanterns are gonna be holy lots o' cheap in the store!
On the subject of Guardians, I just witnessed several of them blasting squid from at least 10 blocks away. So 1) they do not like squid; and 2) they have a very long range, making bows the only logical choice for dealing with them.
FYI: Be well armored. They swim in packs that can be quite large.
Edit: One more thing: Just for giggles, I used Creative Mode to spawn a guardian on land and near water. Sucker flopped around like a, um, fish out of water until it worked its way down to the ocean and plopped in.Suckers like water!
* Prismarine Shards can ONLY be obtained by slaughtering guardians! A Guardian will drop anywhere from zero to two shards! Guardians will also drop Prismarine Crystals in the 0-1 quantity. Remember: You need four shards to craft one Sea Lantern!
** A trick to finding monuments might be the Sea Lanterns. I was able to see them glowing from about a hundred blocks away. Whether that was luck, bug or design I do not know.
*** Elder Guardians can and will target boats!