So, bear with me on this and maybe I can help. Mind you, none of what I'm going to say is going to be easy to do, but that is because your issue is not an easy one to solve. The main problems I see here with any solution is alignment and distance from any mounting point I can discern. My main thought here is you need to get the alternator connected back to the engine and off of the boat. In two of your pictures, I have circled some areas which may be points you can use as mount points for what I'm going to suggest.
In the first picture, there appears to be a water pump with three bolts that are visible. I don't know if these bolts attach to blind holes or go into a water jacket in the engine, so be aware of water leakage. If these bolts are not long enough to mount something off of, you can take a bolt down to a local hardware store (maybe you have a Fastenal in your area) and find a longer bolt with the same thread pitch or find a stud (this is where Fastenal would come in) which is long enough to replace the bolt on the pump side and have the other side protrude which you can attach a brace to. Attaching to two or more brace points (two braces connected or not) would be better.
In the second image, I see possible mount points which appear to be where the engine mounts to the boat itself. Underneath of this, I'm assuming is some type of rubber mount, but the top part, where the very rusty nuts are at, could be used as another mounting point. Also, there is a circle on the back of the alternator showing a bolt in the alternator which can probably used as a mount point on the alternator side to help secure it. In either of these cases, the alternator may be needed to moved or rotated to achieve your goal.
In both of these situations, welding would help, but would not be needed as you can pick up some 1/8" to 3/16" thick metal strapping which can be bent in a vise and drilled to create the right sized holes. You may need to get creative to make something happen, but with a little bit of ingenuity can be done with good results. Alignment of the alternator is key, or you'll be wearing belts out or they will tend to jump off of the alternator. Even if off a small amount can cause issues.
If you still don't believe any of the above will be able to work, my next idea is a lot more wilder and a bit harder to implement.
In the first drawing, you see the engine side pulley on the left and alternator on the right, with the belt running from each. The object in the middle would need to be your spring tensioner. Annotation (1) and (2) are places where threaded rods goes through with a roller bearing in between to allow the belt to ride on. Two nuts on the outside would hold it together. Flat washers would be needed between the nut/plate and between the plate/bearing to allow movement space for the bearing and security of the whole contraption. Annotation (3) is where you'd need to provide a mounting point, to either the alternator itself or the engine, to stabilize the spring tensioner. The second drawing is a close up from side and top of how it would be built. One of the two hole in the plate would need to be slotted to allow movement. This slot would probably need to be fairly long to allow for decent movement of the rod and bearing. You would need to have fairly stout springs on either side.
I don't know where you might get the parts to build this, but it may give you ideas on how you could do this. Let me know what you think.