Pete, did you read what the text says?
Can you see how it may confuse people?
ScubaBoard sends periodic mailings including a newsletter. You are also able to opt-in and receive notifications of new threads or messages. To disable all email messages uncheck this box.
this text seems to imply that other emails and periodic mailings wil be received if the box is checked, not just the newsletter (it says "including a newsletter").
for clarity, you should have a box to check that says:
Check here to receive our newsletter. You will not receive any other emails
or periodic mailings as a result of checking this box.