mjhjmtu

Yo

begintikzpicture
draw (0,0) -- (0,3cm); 
draw (0.5cm,0) -- (0.5cm,3cm);
draw (3cm,0) -- (3cm,3cm);
draw (3.5cm,0) -- (3.5cm,3cm);
draw (0,0) arc (0:180:-1.75cm);
draw (0.5cm,0) arc (0:180:-1.25cm);


draw [fill=blue,blue] (0,0) rectangle (0.5cm,1cm);
draw [fill=blue,blue] (3cm,0) rectangle (3.5cm,2.5cm);
draw[dashed] (0.5cm,1cm) -- (4.5cm,1cm);
draw[dashed] (3.5cm,2.5cm) -- (4.5cm,2.5cm);
draw[<->] (4.5cm,1cm) -- (4.5cm,2.5cm) node[midway,right] $Delta h$;

defRadius1.75cm
defradius1.25cm
 beginscope[even odd rule]
 clip circle[radius=Radius];
 clip[rotate=0] (0,0) -- (0:Radius) arc (0:-180:Radius)--cycle;
 fill[blue] 
 circle[radius=Radius]
 circle[radius=radius]
 ;
 endscope
endtikzpicture

For some reason, the scope isn't on top of my other lines although I have set them all up to start at (0,0).
Any suggestions on how I make this work?

This is because you draw the first shape using arc and the second by drawing a circle. A circle is drawn around its center which you defined implicitly as (0,0), and hence the shift. You can shift the scope by 1.75cm to fit it to the other shapes.

documentclass[border=5pt,tikz]standalone
begindocument

begintikzpicture

fill[blue] (0,0) rectangle (0.5cm,1cm); 
fill[blue] (3cm,0) rectangle (3.5cm,2.5cm); 

defRadius1.75cm 
defradius1.25cm 
beginscope[xshift=1.75cm,even odd rule] 
 clip (0,0) -- (0:Radius) arc (0:-180:Radius) -- cycle; 
 fill[blue] circle[radius=Radius] circle[radius=radius]; 
endscope 

draw (0,0) -- (0,3cm); 
draw (0.5cm,0) -- (0.5cm,3cm); 
draw (3cm,0) -- (3cm,3cm); 
draw (3.5cm,0) -- (3.5cm,3cm); 
draw (0,0) arc (0:180:-1.75cm); 
draw (0.5cm,0) arc (0:180:-1.25cm);

draw[dashed] (0.5cm,1cm) -- (4.5cm,1cm); 
draw[dashed] (3.5cm,2.5cm) -- (4.5cm,2.5cm); 
draw[<->] (4.5cm,1cm) -- (4.5cm,2.5cm) node[midway,right] $Delta h$;

endtikzpicture

enddocument

Edit: I rearranged the shapes so that the blue shape is behind the lines. This way, the lines will be visible.

Here is the output:

This is not an answer but just a comment. You can do that much easier. (EDIT: fixed the water level on the left, sorry, did not pay enough attention.)

documentclass[border=3.14mm,tikz]standalone
begindocument
begintikzpicture
draw [double distance=0.5cm] (0.25,3) -- (0.25,0) arc(0:180:-1.5cm)
-- (3.25,3);
draw [blue,line width=0.5cm] (0.25,1) -- (0.25,0) arc(0:180:-1.5cm)
-- (3.25,2.5);
draw[dashed] (0.5cm,1cm) -- (4.5cm,1cm);
draw[dashed] (3.5cm,2.5cm) -- (4.5cm,2.5cm);
draw[<->] (4.5cm,1cm) -- (4.5cm,2.5cm) node[midway,right] $Delta h$;
endtikzpicture
enddocument

slightly modified code of the interesting marmot's "comment" (+1). it exploit more possibilities of double path option . for exercise :-)

documentclass[tikz, margin=3mm]standalone

begindocument
begintikzpicture[DS/.style=double distance=5mm, semithick, double=#1]
draw [DS=white] (0,3) -- (0,0)
 (3,2.5) -- (3,3);
draw [DS=blue!50] (0,0) arc(0:180:-1.5cm) -- (3,2.5);
%
draw[densely dashed] (0.25,0.0) -- (4.45,0.0)
 (3.25,2.5) -- (4.45,2.5);
draw[<->] (4.35,0) -- node[right] $Delta h$ (4.35,2.5) ;
endtikzpicture
enddocument