Minecraft wiki entry for Multiplayer_PvP_bases, associated text: Using sand or gravel to block off the water, then try to TNT cannon in (which will take a long time and will alert anyone nearby, who will come out and defend against the players)
Ender pearling (which will still take a long time as ender pearls now obey normal Physics and drop straight to the ground once entering water, and if you have your base completely sealed it won't be a problem).
Using sponges to remove water and TNT cannon in (the same ending for situation 1), this method is actually worse than the sand/gravel method, because water sources regenerate and undo most of their work.
Nether Transport (they can enter if they know how to do it, so break your nether-side portal when you're not using it and make sure to hide your portals well). Breaking the inside portal, contrary to common belief, will not deactivate the nether-side portal, and will cause a nether portal to spawn near/on the location of the inside portal.
Find an entrance (which should be well enough hidden that they won't find it).
The only thing you would somewhat have to worry about is multiple players attempting to enter at once, as the guardians' lasers are single-target attacks, and having too many attackers can kill the guardians faster. However, this should not be an issue if you read the above and set up the roof and gotten rid of caves, as if both of these are done, roughly 30 guardians should spawn per second (there is a mob limit still, so you'll only be able to have around 50 at once).