My Fluval-M's have all had high water lines on them. I had one get submersed and it filled with water.
1 out of my 4 Jagers is about 10 degrees low beyond the maximum adjustment.
2 out of 2 of my visi-therms worked for a couple of months and then stopped keeping a constant temperature.
The Fluval M Series is fully submersible. It says "submersible" on the packaging, which would lead you to believe this is the case. Yet I like every other consumer was confused by the water level markings. However, it was easy to verify through a Google search leading to threads like:
http://www.cichlid-forum.com/phpBB/viewtopic.php?t=225268&sid=64ecdea5d480fa4a26dea0b6a7468bbe . I have since replaced all my Slealth heaters with M-Series, have had them all fully submersed since just after the Stealth recall, and have encountered no issues whatsoever. In the original thread I trusted (which I don't have time to track down), they actually posted the response they received from Fluval (Hagen). It mentioned that the reason that both high and low markings are on the heater has to do with specific safety regulations in certain areas that the heater is sold. It's kinda like every product sold now has to say that California knows it to be harmful and cause cancer. But Fluval really did drop the ball by not explaining things better on their packaging/instructions, especially with the heaters they shipped after the Stealth Recalls, as that had to have been a huge boost in sales, just from PetSmart alone!
Sorry to hear you had bad luck with yours, but it must have been a defective heater in general, as opposed to a problem that occurred just because it was submersed.
That being said, unless a company treats me really good when I contact them about a defective product, I rarely give that same product another shot, and can totally understand you not wanting to do the same. I just wanted to be clear for everyone else that the M-Series are in fact fully submersible.
Thanks blue!! I will check the marineland visi-therm too.