For my boat using a lead-acid battery, I use an Battery Tender 800. These are designed for leaving connected and float-charging indefinitely if you want. I confirmed it charges and floats correctly and have used this model for well over a decade connected for months in some cases (accidentally) so I know the battery isn't boiled out or damaged by it.
But that's not how I intend on using it, for various reasons.
For my Seadoo boat, I installed a stainless marine cigarette lighter live outlet at the helm that I use for powering water toy air pumps, etc. and occasionally, a 100W spotlight. This live socket is connected directly to the battery +/- terminals with it's own circuit using 12AWG (20A capacity) marine wire and fused inches from the battery + using a marine 15A blade fuse holder.
So during winter before covering the boat, I turn off the battery switch and plug a 10ft battery tender charging cord into this live 12V outlet and leave the cord hanging outside the hull. Periodically (once a month or less), I connect the battery tender to confirm the battery condition. Usually the charger quickly goes to float mode within 30 seconds and I disconnect it. If not, I leave it connected and recheck a few hours later or next day.
This way, I'll know if some problem with the battery might be occurring. A lead-acid battery should hold it's charge quite well for over a month, most often for a few months. Say for instance the battery develops a shorted cell, it wouldn't charge correctly and this method will immediately reveal some problem exists.
This frees-up this same float-charger as well, for use maintaining other toys in a similar method.
Anyway, I prefer not leaving stuff connected to AC power when it's unnecessary. Maybe b/c I've witnessed what has gone wrong for others a few times and have experienced other battery chargers fail with less than stellar results.
And I've had lead-acid batteries last upwards of 15 years when maintained using this recipe. New batteries do fail as well, this season I witnessed 3 failed new batteries.