The fluval M series is a good heater. For any heater you need about 3-5 watts per gallon of water. If your 50 long is filled up all the way, I'd aim for a 200 watt heater.
The fluval M series are decent heaters. They are impact resistant borosillicate glass tubes according to the fluval page. Which to me could just as well say "they are made from flimflamkabaob" as I have no idea what borosillicate glass is. So they are glass of some sort, keep an eye on the heater if your turtle likes to mess with it.
The fluval M series is 100% submersible. If you call hagen fluval they will tell you so. There are people on other forums that have confirmed these heaters are fully submersible.
The problem with the fluval heater is that it's deisgned and manufactured in italy. Like all things designed and made in italy, it will look incredible, be an engineering marvel, have a ton of features, and fall apart in a week. The exception to the rule is italian clothing and leather goods.
http://www.fluvalblog.com/product-spotlight-m-series-heaters-video/