Inflatable enema plugs (such as: ) will certainly help you stretch out your anus. I have one that used to be attached to enema pants (which were amazing). It's stand-alone now, much like the picture above (I think the petrleum products ate the glue the bound it to the rubber enema pants).
If you inflate that to it's full capacity it's gets to about a 4-inch diameter. It is pliable and baloon like (the air will 'squirt' to other sides of the balloon when you try to circle your hands around it and squeeze), but still, that's at least going to come out of you, under pressure (from the sphincter) at about 2.5-3 inch diameter fully inflated. The more you work out your sphincter that day, the larger it will start coming out. It's quite a good trainer.
This is very different than the Doc Johnson style inflatable butt-plugs. Those are very thin rubber. It feels like a fingernail can pop them (but they don't, and do last). This is a much heavy grade rubber.
The feeling of it coming out is euphoric. Not to mention, when it's pushed out due to a peristalsis action, you'll come close to passing out in the tub.
Playing around with one of these while giving yourself enemeas in the tub will walk you up the anal-toy diameter ladder. You generally move in quater-inch increments. It's an amazing feeling.
The toughest thing about water-play and stretching is finding the right lubricant. You pretty much need to go silicon and deal with the slickness after the fun, or petroluem and later clean your rubber toys and wipe them down with mineral oil.