Glenn, I'm no expert on sea spiders, but my impression is that they are
small, slow moving animals, generally with little color, and pose no threat
to humans.  They would have little use for a venom apparatus, given the
small, soft, sessile nature of their prey.  The few I have seen have been
under 1 inch in length.  Of course there could be larger species.  But what
you describe sounds more like some sort of arrow crab or spider crab.
Paul M.