6-71s are dry-fit liners. That is, the block has the cooling passages in it, and the liners are an interference-fit into the bore. There is no direct coolant contact with the liner; heat transfer is by metal-to-metal contact. When these engines are overhauled they MUST have a large number (more than 20!) VERY precise micrometer measurements taken, the correct (based on those measurements) liners ordered, and the fitting process must be followed TO THE LETTER.
If that is not done, you will get localized overheating due to poor heat transfer in the new liner to block. The result will be a trashed hole, usually evidenced by low compression in the affected bore - the evidence is apparent when the head comes off, as the liner has a blue color to it where it overheated and/or it is seized in the bore and won't come out! If you ever find a 6-71 with that kind of damage and it has been overhauled it is proof that the overhaul was botched. Getting these measurements in a clean enough environment to be precise enough, and doing the fitment correctly with the engine in the boat is POSSIBLE, but not easy.
There is no "seal" per-se on a dry-fit liner to leak coolant. If your engine is losing coolant, it is likely an external leak or coming from a seal in the head.
6V92s (the V92 series generally) have WET liners. The liner is cooled by coolant that passes around the liner; the block and liner do not directly touch except at the top where the lip of the liner fits into the deck. The liner is sealed from the rest of the block by elastomer seals (basically big O-rings) at the top and bottom of the bore. These seals are made of material that is impervious to attack by antifreeze and oil, BUT like all elastomer materials, it is subject to damage by excessive heat. When that happens you almost always get coolant leaks into the oil (from the bottom seal, which has the crankcase on the other side of it.)
BOTH designs can leak due to overheats in other places. Overheats are particularly bad on Detroits due to the very complex block and head castings. Heat transfer is critical, and its very easy to end up outside of design limits, particularly with high performance "recreational" marine engines that are running on the ragged edge of permissible temperatures internally anyway. This is a function of the 2-stroke design, which must pass air through the airbox into the cylinder ports, and the exhaust ports in the head, plus the injector tubes. The injector tubes, made of brass, are a press-fit (with seals) into the cylinder head. Overheats (or overtorquing the injector nut hold-down bolt) can compromise THOSE seals as well. The head has coolant flowing through it, as well as oil, and compromised seals there can leak coolant into the oil, oil into the coolant (!), or either into the engine where it is burned and expelled. The seals most often compromised are at the injector tubes, although the head-to-deck seal is also at risk.
Be very careful of coolant leaks. If you can't FIND the coolant (e.g. its not showing up in the oil) then it is PROBABLY going into the cylinder and/or airbox. Too much of it when you're shut down can lead to a hydrolock, which will immediately destroy the engine on startup. Further, even small amounts of coolant can do severe cylinder and ring damage due to both glazing and rust (from the water in the coolant) if it sits.
Another possibility is a leak in the exhaust manifold(s); on most Detroits these are in the freshwater circuit. Most small leaks there do not end up trashing the engine as the manifolds "hang down" from the exhaust ports, and thus, small leaks don't go back into the engine. The bad news is that you REALLY won't like the price on a replacement.
Bottom line is that you must find coolant leaks and STOP THEM, no matter the source. Nearly all of the possibilities can lead to a trashed engine through one mechanism or another.
