I'm thinking, if the players place the phylactery at the bottom of the ocean or weight it down in a sea of lava somewhere. Would the lich be able to be rejuvenated at the bottom of a sea of lava or under the pressure of the ocean floor? What about dropping it into the Sun? What would happen to the lich if that doesn't destroy the phylactery (magically protected) but wouldn't be all too convenient to reappear within a few feet of?
RAW, the lich would reform
The lich's stat block describes rejuvenation (MM, p. 202):
Rejuvenation. If it has a phylactery, a destroyed lich gains a new body in 1d10 days, regaining all of its hit points and becoming active again. The new body appears within 5 feet of the phylactery.
No mention is made of what would happen if there is no unoccupied space within 5 feet of the phylactery, of whether anything else might block it (pressure on ocean floor, etc), so RAW, it simply reappears there. This is also not expanded on in the lore (MM, p. 203):
Death and Restoration. [...] Within days, a new body forms next to the lich's phylactery, coalescing out of glowing smoke that issues from the device.
Nothing more is said about it, so unless the DM were to rule that it would not be possible (such as if it were surrounded on all sides by 10 foot thick walls with only a tiny pocket for the phylactery in the middle, and even then, the DM may rule that it simply waits until it does have room, or that it reappears in the nearest unoccupied space, etc), the lich would reform.
For what it's worth, I am as DM would rule that it would be able to reform at the bottom of the ocean, that the magic of the phylactery somehow ignores the water pressure, but if it were in a small pocket of space (i.e. only just enough room for the phylactery, definitely not enough room for a lich) surrounded on all sides by at least 10 feet of solid wall, then it couldn't reform and would have to wait until there was room (i.e. walls moved/destroyed or phylactery magically removed).
But if the environment would kill it, it would be locked into a cycle of death and rejuvenation
If, for example, we assume that the phylactery was at the bottom of a pool of lava, and if we assume that the phylactery is unharmed by this lava, then unless the lich has a way to magically escape its surroundings before it loses all of its HP, then it would die and reform again in the exact same spot... locked into a cycle of death and rejuvenation until the lava pool stopped existing somehow or the phylactery was somehow removed from the lava pool.