When attaching a spearfishing reel to your speargun, you've got two main options, vertical or horizontal reels. The differences between the two are fairly minor and ultimately, like many things in spearfishing, really comes down to personal preference.
A vertical reel is better aligned with your speargun, but because of it's positioning creates a little extra drag when tracking through the water. The advantage of this design, however, is that your reel line runs closer to the barrel than on a horizontal reel. This means less opportunity for your line to snag on the reef, and less drag from your line. Ultimately though these things are subjective and vary from reel to reel, so do some reading, or come into one of our stores and chat to our experienced staff.