Submersible powerheads and heaters ???

[IluvHockey]

On the power heads the stickers read do not immerse beyond the marked water line. On the visitherm heater there s a minium water mark but they also show with a red arrow not to immerse pass the start of the rubber top. So I don't think this is so. What do you do? Do most people follow these intructions or do people even read the fine print? This stuff is new for me.