A player whose disc is declared lost shall receive one penalty throw. If the throw was made from the tee, the player will re-tee for the next shot. If not made from the tee, the group will determine the approximate lie from which the throw was made, and the player will throw again throw from that lie. In all cases the original throw plus one penalty throw shall be counted in the player�s score.

In 2006 let's say I throw a disc 250 feet across a pond which has been declared out of bounds (shoreline defines the OB line). The disc hits the ground and rolls into the murky water. We walk around the pond, look around where the disc was last seen rolling into the water, but it's gone. The disc is not retrievable, nobody can see the disc in the water.

I have to go back to the tee and throw again, right? (And if we know in advance the water is murky, throw a provisional from the tee in the first place.)

No. If there's a reasonable chance the disc is OB, that rule takes precedence over the lost disc rule. The rule priority is like this:

Missing a mando
OB
Lost Disc
2-meter rule (if enforced)

and these are not added together. Just a 1-throw penalty whichever one applies.

The murkiness of the water is irrelevant. You don't have to see the disc in an OB area. You can but don't have to go back to the tee as one of your OB options, but not due to lost disc rule.

The language of the rule in 2006 will say that if there is "reasonable evidence" that the disc was lost out of bounds, you take the penalty for out of bounds.

803.09 A - ... In order to consider the disc as out-of-bounds, there must be reasonable evidence that the disc came to rest within the out-of-bounds area. ...

I see it now. Thanks guys!