Wood is good!.. 
Having nearly been crushed under cars and trucks a few times in my younger days,I now wont go under anything without it being blocked up or supported by jack stands,or wood blocks--and I leave the jack up aganst the vehicle's frame too,in case it decides to fall sideways and off the stands..
I find any jack stand extended over 18" to be VERY unstable and tippy,and you risk having the vehicle fall on you when tightening things like U bolts by hand with a breaker bar..my first car,a VW Beetle,fell on me while I was pulling the last bolt from the bellhousing when I was pulling the motor out to replace the clutch!..
Luckily it only pinned me down,it didn't crush me!...one block of wood I had it propped up on jammed between the brake drum and the asphalt,and my friend quickly jacked the car up off of me..face looked just like the bottom of the engine--I had FINS pressed into my skin,black eyes,bloody nose,and a LOT of pain!..it only takes ONE time for this to happen for you to vow to never let it happen again,but as you age you get lax,and take more chances than you should..it took 3 more close calls before I finally decided to use something better than just jacks and jack stands..
I wont go into detail on the cars that FELL off the forkloder at the boneyard,when we'd cut out rear ends and engines..
..more than once I nearly got squashed!..we used it for a "lift" to fix vehicles too,and though it did work out well,its scary as hell to be working under a vehicle while its inly held up by two forks and gravity,especially when your pulling a heavy part like a tranny out,that can upset the balance when its removed,and make the vehicle tip off the forks!..
What I've used when I need to support a vehicle higher than my jack stands will,is huge chunks of pine,cut from trees about 2' in diameter with my chain saw,with the "logs" cut as squarely as possible on each end,to reduce "wobble" when the vehicle is on them..
I've used them as high as two feet with no sign of unsteadiness,whereas jack stands would let the vehicle sway in a strong wind at that height!..the wider the base of the log the better,and hardwood would be better than pine if available..square cut timber works good too,and can be stacked like cribbing to support a vehicle up high,like when rolling a chassis out from under a body..easier to store and carry when its not one huge log too,but I feel the logs are the safest..
I've seen guys make huge "sawhorses" out of big timbers or channel iron too,to hold a vehicle high off the ground..they seem to work well too,very stable and safe..clumsy and heavy though,and hard to store..
I'd never trust any sort of concrete block or bricks to support something I'm going to be working under--those who have and gotten away with it are just plain LUCKY...not saying they wont work,they do--but they CAN shatter without warning when subjected to "point loads"--like when only a small portion is supporting lots of weight,like a bolt that happens to stick out and touch the blocks surface and has to support most of the weight on that one spot,can cause it to crack or shatter..on a boat this likely isn't a problem,you have a full flat surface for the blocks to rest on,so failure is less likely..but you wont see me crawling under one supported by cement blocks..no thanks!
Just my 2 pesos worth.
And it's way stronger...
Not sure if the pre-fab is much stronger either... I've hefted plenty of cement blocks and none were near as heavy as mine are. I dunno, maybe I had the super 'crete or something. 
