Lava Is a Low-level Threat
Pathfinder's rules for lava say that
Lava or magma deals 2d6 points of damage per round of exposure, except in the case of total immersion (such as when a character falls into the crater of an active volcano), which deals 20d6 points of damage per round.
Damage from lava continues for 1d3 rounds after exposure ceases, but this additional damage is only half of that dealt during actual contact (that is, 1d6 or 10d6 points per round). Immunity or resistance to fire serves as an immunity to lava or magma. A creature immune to fire might still drown if completely immersed in lava (see Drowning).
This means that after somehow gaining but 1 point of fire resistance, lava is as dangerous as murky water.
Although one still must find a way to breathe while submerged and make Swim checks, the lava problem itself is solved by, for example, the 2nd-level spell resist energy. Cast the spell resist energy on a party member then have him hold his breath, dive in, and feel around until the item's located.
If the DM has house rules regarding lava that changes these rules--and, perhaps, most DMs should--the DM should make those house rules clear to the players.
This particular rules quirk has existed since Pathfinder's grandfather Dungeons and Dragons, 3rd Edition, whose Dungeon Master's Guide (2000) says, "An immunity to or resistance to fire or heat serves as an immunity to lava or magma" (89). If an editing error (which it really could be), it's a 14-year-old editing error that neither Dungeons and Dragons 3.5 nor Pathfinder corrected.