If I am staying on Lac La Croix in the Boulder or Tiger Bay area, and want to get to Bottle or Iron, can I take the 88 rod portage from LLC to Bottle on the Canadian side, or do I have to take the longer 326 rod portage from LLC to Iron? Can I just pick my way around the rapids on the Bottle River? I'm interested how the fishing is in the confluence between Iron and LLC in May. Any other day trip fishing ideas are also welcome!

Per the Webster-Ashburton Treaty - 1842, you can take the shorter Bottle - LLC portage without concern of either a RABC or creating a border infraction. We did that on a BW trip during Covid coming from the Crooked direction. Presuming you're not dilly-dallying or fishing/camping out on the Canadian side, you should have no issues. I've not fished any of the area, but moving water is always prime territory to cover. I couldn't tell you anything about picking your way through the rapids but feel pretty confident in saying you'd be better served timewise to take either of the portages, particularly if the water is high.

Have stayed in that area several times. Take the short portage vs the long one, just trust me.

As far as fishing goes, it is usually good but last year we had a weather system come in right as we got there and the fishing shut off. We fished at the bottom off the rapids, walked all the way up to Bottle Lake, which wasn't easy, and fished the top. We fished all the way from the bottom all the way out to LLC and the only thing that was biting was Snot Rockets. We caught a few decent ones but nothing picture worthy.

As mentioned you can take the shorter portage. However, I would note that the shorter portage actually splits about a quarter of the way in. Staying on the main trail is a little shorter and doesn’t get as high, but it can get very wet and muddy. Take the higher route to the right. You climb in elevation, but it is definitely dryer. The left route basically follows the valley.

Gone that way to Iron probably 7-8 times.
Legally your fine as mentioned.
Years ago , Bottle Portage didn't have the 'right' option and could be boot sucking mud. Or at least that I knew about. If it's wet, for sure go right. The problem in dry conditions is the left option may extend the port to the left another 20 rods or so in not so perfect conditions.

Don't take that long portage, it is usually pretty nasty. It is better to take the Canadian side portage in spite of being more paddling, and like previously stated, When it splits, take the high road(On the right I believe). If it is extremely windy the long portage can be the better option.

Depending on which camp site you're at (i.e. southern end of boulder bay vs northern end of Tiger Bay) the longer portage can make more sense. The longer portage is also two portages split up by a large beaver pond, roughly 65 rods on the west side, 240 rods on the east side. If you end up going this way, there has been a birch bark/paracord marker where the west side of this portage starts in the beaver damn as it can be difficult to spot. As far as the portage goes, it's not overly difficult just longer. Per your post however, if you're planning to explore/fish the bottle river/lake then certainly take the bottle portage.

Thank you everyone, much appreciated!

I was in this area last year. Very enjoyable. The rapids are no passable by canoe and we did not find them "pickable" either. Fishing We took the longer on the Iron side was fun and we did okay in that moving water. We took the longer portage around and it was long, a little muddy (especially in the middle pond part) but we did not suffer too greatly and after we got to the other side we looped back towards the mouth of the river where it comes in to LLC and one of our guys caught the biggest walleye of the trip right there.

I suppose in reading this we did not need to go the long way but we were fine. Such a fun area.

We have been this way many times with an early start at Moose River North and pushing on to Boulder Bay or Tiger Bay on day one. Agnes is just to busy for me. My sons would reather portage then paddle so we have taken the long portage route several times with the beaver pond in the middle. Getting to the beaver pond is easy but finding your way out is harder, The portage landing going east is hidden pretty good. Unless somone has flagged the entrance to the landing you will be looking around awhile. This is where a GPS comes in handy. As mentioned in previous posts, the Bottle portage can get messy with mud but you should know it's condition before you get there by the shape of the other portages you have humped over before you get there. The alterative portage trail to the right is a bit of a climb but avoids the mud. If the water level is low expect the Bottle Lake end of the portage to be nasty, You may have to do some rock hoppping to get to some canoeable water,

BTW: I think Iron Lake is a fish factory. Lots of moving water. I've caught 6 species of fish on that lake in one day. The island campsite on south end by Peterson Bay is a gem.