Is there something to put between the pump and motor to keep the sand out? How much, can I make one? Put removeable freeze plugs in and hose out every so often?

I did have a small screen filter in line and sucked up sand a few times ,it caught it but also pluged up and stoped the water,keep an eye on your temp.Now I dont run a filter,just watch my sand bars.....

"Somebody told me that Chevy block's have plugs that can be removed for helping to clean out debris, right above the oil pan at the bottom of the cooling passages. I don't know if other man.'s are like that or not...  Anyone else know/heard of that?

Yep, the BBC and the Olds have those plugs. Looks like you would just need to taje the plugs out and run water through it. 

How nessasary are they I wonder??? You are always going to have some sediment, and with the water flowing it would seem like it would just keep flowing on out. Maybe I'm wrong though...

With the enormous amount of Water (along with sand and sediment)  that a jet drive boat "processes" its enevitable that there will be a "sand" collection in the lower reaches of the block due to gravity settling ,although the ability to drain at 3/8 petcocks or plugs is present in all of the popular motors (Ford Chevy and Odls for sure) its doubtful that much will come out (weight) I wont disagree that most Jet boat motors will have a fair amount overtime but we truthfully dont see this as a big problem on a regular basis. We have gone so far (during core plug replacement ) as to "vacuum" out the block coolant passages in the jackets above the pan rails . Its especially important during building to use brass core plugs as the moist sand setting behind the plugs (especially fords) will accelerate the rot/rust Not a huge fan of Filters and strainers here btw  Just my 02 Tom

Remember watching the temp gauge does nothing if no water is flowing through. Without water there is no temp reading... Don't use a strainer. Just try your damndest to start your boat in minimum chest deep water. That'll prolong the life of your wear ring, impeller, etc, etc and keep just that much more sand out of the block.

Good advice ! also on an somewhat related note IMO the most important temp you might want to monitor on your Jet boat motor is Oil temp ,depending on the setup water temp is somewhat meaninless as the water passes thru too fast in most cases to effectively absorb and "carry off" the heat Tom