Can we add a Terms of Service thing for members to agree to before they register?

yes you can enter them in a topic and invite the news members to ack. and aprove it.

Is that the only way?

yep.
lots of admin do this way.

however, when registering, new members must check a box indicating they aprove aceboard's TOS

Bummer I was hoping that we could add a link for them the agree to Sailorsbetterhalf TOS

the only option is :

public secrtion of the forum with the TOS where new members post their agreement

then you allow them in the private sections.

for this, your sections must be defined as "private (reserved for members designated by the admins)

alright thank you