First, On what body of water do you live on? and how did you fit the Piers into it?
are you going through the PC with the cable modem attached with a Xover cable to a second NIC?
If so,, then the 2nd PC should have a private IP,, I am thinking that you have the XP to the cable modem and the 98 with the Xover...
check the IP on the 2nd PC,, I am thinking that it is probably a 192.168.x.x number with the XP box IP as the Gateway..
with the firewall running on the first PC, I would say that the 2nd pc is in most cases safe,, however, you get a virus that disables that firewall and there is another story,,,
I would change the IP on the 2nd PC to an address in the same scope but higher and odd,,, in this way it would not be so obvious in the event that someone actually got through,,
when you are live on the net,, I dont believe that anything is TRULY safe,,
just some thoughts,,,