No @123vca. you do not have to accept your admissions to apply for housing.
Have you been admitted to Galveston yet? You should have a links on your AIS that takes you to the housing portal for Galveston as well as your next steps.
Here are your Next Steps: https://engineering.tamu.edu/academics/engineering-at-galveston/next-steps
Here is another link that will be helpful
http://newaggie.tamu.edu/wp-content/uploads/2017/02/EA-Galveston.pdf
Galveston Housing FAQ: http://www.tamug.edu/reslife/Housing/Housing%20Application%20and%20Assignment.html
Here is the apply to housing link and info you need to read: http://www.tamug.edu/reslife/Housing/Apply.html