If its the great pyramid of Giza, then the stock lens might be fine, but given the "...telephoto..." comment I suspect that this "structure" isn't that large. One of the things that is difficult to achieve at very long distances is the avoidance of the atmospheric effect. A few more details would likely be helpful.
Unfortunatly, given the distance that you are discussing - you will probaably want to get closer to prevent atmospheric issues. But of course shooting from a (small) boat can create all sorts of chalenges!
Though a good place to start after you hit the stop on your zoom would be a 2x zoom externder adapter to give you some more reach.
thanks, mikko. the 2x seems worth a try - seems like the one everbody reccommends is the century schneider optics converter, which i would love to get, but have to save up for that - about UK£900. maybe hiring a small boat might be worth a try
The Schneider Optics is the ONLY solution for the HVX. I use the 2x for these sort of shots, and on a boat! Shooting from a boat is a trick, but having done it for months now, I have kleets and stuff all over mine. People freak out thinking I'm doing home made voodoo fishing rigs! LOL! I'm like, no dude, it's for my cameras!
The Schneider lenses vignette bad, too. You only have the last half of the zoom range that is useable. From what I understand, on both the teleconverters they sell. I use the 2.0x, I haven't used the other one. I did start a post on this sometime ago, too.