When you have a small leak in a non-critical area, such as on the floor, or the transom, 5200 is acceptable. But, when you're dealing with an issue like a stress crack in a weld seam, 5200 is only a band-aid, temporarily masking the problem until the crack propagates and opens up more, causing an even bigger leak, or worse.
And on a seam such as where the transom meets the floor, or the floor meets a gunwale, there are a lot of stresses at work.
Best course of action (at least this is how I do it when people bring boats to me for welding)
I use the torch, and slightly heat up the area of the crack, heat will cause expansion and will show the full extent of a crack. It's easy to miss a hairline crack propagating outwards from the main area, and that hairline crack will eventually cause an issue. Also, heating, and CAREFULLY hitting the oxygen lever on the torch a few times, will burn off and oxidize any contamination that has crept into the seam, especially in an area like the transom/bilge. Failing to do this will allow those impurities to contaminate the weld.
Stop-drill either end of the crack with a small drill bit, OR, using a cut wheel on a grinder, vee out the crack with about a 60 degree bevel, making the cut deepest at either end, and going at least 80% of the depth of the material, this allows for a good solid weld.