I wanted one that was easier to print, without supports, and has fin options and the ability to adjust the ballast and center of gravity.  I meant for the fins and nose to be printed in TPU for durability, everything else is your choice.  I have a included a hollow midsection that can be filled with whatever is handy.  

To get the density such that it is heavier than water, make sure the sliced weight in grams is greater than the volume in cubic centimeters (if it's in mm, then 5,683=5.683 cm^3) since water is 1 gram per cubic cm. (see the screenshot from my slicer).  It took me about 11 perimeters and 50% infill for the midsection, so just keep going up until you get the weight there.  

To adjust the tightness of the threads, adjust the X and Y scale only on the threaded connectors.  If you scale the model up, you will have to go up even a little more on the X/Y of the connectors (at 200% the connectors X/Y should be at least 205%).  I recommend printing these in TPU as well for durability since it has great layer adhesion and tight tolerance forgiveness.  You may need supports on build plate only for the connector holes above 150% but I left some clearance for bad bridging in the holes.  

Enjoy!!