2 questions

[Braininthejar]

1 can a tattoo work as a holy symbol for the purpose of spellcasting?

2 a summon spell can only summon creatures on surfaces that can support them. Is it possible to summon a fire elemental up among the sails of the ship (on the ropes the crew normally uses to climb the mast?)

[Jackinthegreen]

There are no rules for making a holy symbol out of a tattoo.  You'd have to homebrew it.

Summoning on some rigging would probably work, but it's up to the DM to determine whether that would work because there aren't any rules interactions for that specific situation that I have ever read about.

[Kethrian]

1.  As Jack said, it would need to be houseruled.  But I personally wouldn't have a problem with it.

2.  You could summon it onto the yard arm, and have it jump down onto the sail....