I'm 18 and I have a very slight underbite. Could it still get worse and can I still correct it without surgery if I've already have braces?

Check growth. If problem slight braces may correct underbite. If more intense, braces & jaw surgery may be required. At 18 you may still have more jaw growth to go. An orthodontic specialist can track this for you. Seek a qualified evaluation, understanding that lower jaw growth should be entirely completed before jaw surgery.
Not get worse. It will probably will not get worse. Please see an orthodontist, with a solid reputation!
Orthodontist. I would discuss this with your orthodontist, they should be able to address without surgury if your case is not severe.
Possibly. Even though you are 18, you still may be growing. If your lower jaw continues to grow, your slight underbite may actually get worse. Once your jaw growht has ceased, then and only then can it be determined whether surgery is needed.
Underbite correction. Many guys jaw, feet, hands & bones that form in that manner grow into their early 20's. Your underbite may worsen during the next few years. In some cases an orthodontist can help minimize the progressive growth, sometimes not. The surgery to correct is no picnic. I would only have the surgical treatment done if you are 100% you are finished growing and you have significant dental symptoms.
Hard to say. At age 18 your teeth are unlikely to change much on their own so your underbite will likely stay where it is, but it can have a negative effect on how your teeth wear and function over time. Having braces once doesn't mean you can't have them again, but without seeing you personally, there is no way to say whether or not surgery would be needed. Try getting a consult at an orthodontists office.
Ask Orthodontist. Are you currently under the care of an orthodontist? Was your orthodontics completed? How long ago? Were you given retainers? Have you been wearing them? Your question is best answered by an orthodontist who can evaluate your current occlusion after personally examining you.