Why didn't Voldemort just drop Salazar Slytherin's Locket on the bottom of the Horcrux cave so that when someone was trying to get it and destroy it (Regulus, Harry, Dumbledore),they'd have to swim down deep, only for them to get drowned by hundreds and hundreds of Inferi, thus making their attempt futile at the very least?
Inferi aren't a big deal for an experienced wizard. Fire and heat always works even under water (remember, inferi actually took Harry under water, but Dumbledore's fire acted on them even under water). And, you can survive under water using gillyweed, and lots of spells.
Other than that Voldemort chose those locations where he himself could easily reach.
He didn't want to kill any intruders without first finding out how they learned of his Horcruxes.
"Oh, I doubt that it would work like that," said Dumbledore easily. "Lord Voldemort would not want to kill the person who reached this island."
Harry couldn't believe it. Was this more of Dumbledore's insane determination to see good in everyone?
"Sir," said Harry, trying to keep his voice reasonable, "sir, this is Voldemort we're-"
"I'm sorry, Harry; I should have said, he would not want to immediately kill the person who reached this island," Dumbledore corrected himself. "He would want to keep them alive long enough to find out how they managed to penetrate so far through his defences and, most importantly of all, why they were so intent upon emptying the basin. Do not forget that Lord Voldemort believes that he alone knows about his Horcruxes."
Voldemort would have wanted to be able to access the Horcrux if he needed to. If it is at the bottom of a lake he would have struggled to get to it.
It is also worth remembering that Voldemort is exceedingly arrogant. He didn't believe for a second that anybody would ever find out about the Horcurxes.
Finally, I believe that Voldemort would have considered it impossible for anybody to get to the locket thanks to enchantment on the boat. He would have believed that it was impossible for two witches/wizards to get there and because he believes that those who love are weak, he would believe that somebody wanting to destroy the locket would have been too "good" to use a house elf or other such creature to drink the potion.