We stayed at Turneffe last year and dove the Elbow every day for a week! It is the most amazing dive site and well worth the travel. If you are concerned about seasickness, contact your physician and ask them about oral Zofran. This is a medication that can really help with the nausea and doesn't make you feel weird like Scopolamine patches do. You can take Zofran and still be OK to dive.
Happy diving!