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I have the submersible Marineland Stealth 100W heater. The way I have it set up now the heater is only about half way under the water. Will it still be able to heat up the tank or do I have to put it in all the way?

You should put it in all the way or so only a little at the top is above water. There's a line on it near the top indicating the "low" water line. If it's too tall to submerge in a vertical position, you can angle it or even attach it horizontally.

Personally, I like to leave the top of the heater where the electrical cord attaches above water because my turtle sometimes wants to bite the cord when it's under water.

The Stealth is fully submersible, but it's important to read the directions/instructions that came with it.

You should at least have it submerged up to the "minimum water" line. If it works as advertised, it will shut off if out of the water, and having half of it out of the water could very well cause it to shut off. If it's not working correctly, it won't heat the water.

so its ok to totally submerse the heater? i bought a package deal for my tank so the heater didn't come with any instructions..also this might be a stupid question but is it safe for my res to touch the heater? or do i need to place the heater in a spot where it cant get to it?

Does the heater say it's fully submersible? What brand and how many watts is it?

If the heater is submersible and it's under water, it should be OK for your turtle to touch. The heat quickly dissipates in the surrounding water, so I don't think you have to be concerned about your turtle burning himself. If you're still concerned there are heater guards you can buy, or you could make one yourself out of PVC pipe.